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# Surface Irrigation

## Summary and Keywords

Surface irrigation is the oldest and most widely used irrigation method, more than 83% of the world’s irrigated area. It comprises traditional systems, developed over millennia, and modern systems with mechanized and often automated water application and adopting precise land-leveling. It adapts well to non-sloping conditions, low to medium soil infiltration characteristics, most crops, and crop mechanization as well as environmental conditions. Modern methods provide for water and energy saving, control of environmental impacts, labor saving, and cropping economic success, thus for competing with pressurized irrigation methods. Surface irrigation refers to a variety of gravity application of the irrigation water, which infiltrates into the soil while flowing over the field surface. The ways and timings of how water flows over the field and infiltrates the soil determine the irrigation phases—advance, maintenance or ponding, depletion, and recession—which vary with the irrigation method, namely paddy basin, leveled basin, border and furrow irrigation, generally used for field crops, and wild flooding and water spreading from contour ditches, used for pasture lands. System performance is commonly assessed using the distribution uniformity indicator, while management performance is assessed with the application efficiency or the beneficial water use fraction. The factors influencing system performance are multiple and interacting—inflow rate, field length and shape, soil hydraulics roughness, field slope, soil infiltration rate, and cutoff time—while management performance, in addition to these factors, depends upon the soil water deficit at time of irrigation, thus on the way farmers are able to manage irrigation. The process of surface irrigation is complex to describe because it combines surface flow with infiltration into the soil profile. Numerous mathematical computer models have therefore been developed for its simulation, aimed at both design adopting a target performance and field evaluation of actual performance. The use of models in design allows taking into consideration the factors referred to before and, when adopting any type of decision support system or multicriteria analysis, also taking into consideration economic and environmental constraints and issues.

There are various aspects favoring and limiting the adoption of surface irrigation. Favorable aspects include the simplicity of its adoption at farm in flat lands with low infiltration rates, namely when water conveyance and distribution are performed with canal and/or low-pressure pipe systems, low capital investment, and low energy consumption. Most significant limitations include high soil infiltration and high variability of infiltration throughout the field, land leveling requirements, need for control of a constant inflow rate, difficulties in matching irrigation time duration with soil water deficit at time of irrigation, and difficult access to equipment for mechanized and automated water application and distribution. The modernization of surface irrigation systems and design models, as well as models and tools usable to support surface irrigation management, have significantly impacted water use and productivity, and thus competitiveness of surface irrigation.

# Introduction

## Surface Irrigation Methods

Surface irrigation systems (SI), or gravity irrigation, are those systems that, unlike pressurized systems, depend only upon gravity to convey, distribute, and apply water on the land surface. They developed after millennia, through ancient civilizations like the Harappan in the Indus basin (Ratnagar, 1986) and the Egyptian in the Nile basin (Butzer, 1976), as well as the civilizations of China (Tan et al., 2005), Mesopotamia, and Near East (Bazza, 2007) and the Pre-Colombian Americas (Martínez Saldaña et al., 2009). Surface irrigation largely contributed to the development of hydraulics engineering (Viollet, 2007); related hydraulic structures and field practices continue to be used at present (Zhang et al., 2013; Chipana-Rivera et al., 2016). Ancient developments are the origin of a worldwide variety of surface irrigation methods and systems in use at present, which are adapted well to a variety of crops and crop systems. They have been modernized in several regions in Southern and West Asia, the Mediterranean basin, and North and Central America, or are being replaced by mechanized and often automated pressurized systems, which are less demanding in terms of management and labor. However, surface irrigation is the most common worldwide with the exception of Europe (Table 1).

Table 1. Estimated Irrigated Areas by Irrigation Method and Respective Fraction Per Continent, Regions, and Total Area Equipped for Full Control Irrigation and Its Fraction of Total Cultivated Area.

Continent regions

Area of full control irrigation (Mha)

Irrigation method fraction (%)

Total irrigation equipped area (Mha)

Irrigated land in % of total cultivated

Surface irrigation

Sprinkler irrigation

Micro- irrigation

Total

Surface irrigation

Sprinkler irrigation

Micro- irrigation

EUROPE

29.93

63.31

6.76

21.40

7.30

Western & Central Europe

3.69

6.69

0.95

11.33

32.56

59.04

8.41

16.57

13.50

Eastern Europe

0.59

2.37

0.02

2.98

19.93

79.57

0.50

4.81

6.80

AFRICA

65.27

20.27

14.47

15.60

5.80

Northern Africa

4.65

0.92

1.13

6.70

69.36

13.74

16.90

7.40

25.60

Sub-Saharan Africa

2.30

1.24

0.41

3.95

58.32

31.35

10.33

8.20

3.40

AMERICAS

58.09

35.72

6.19

52.20

13.10

Northern America

17.25

13.77

2.00

33.02

52.23

41.71

6.05

34.30

14.90

Central America & Caribbean

1.28

0.26

0.09

1.63

78.99

15.73

5.27

1.80

13.00

Southern America

9.80

3.39

0.94

14.13

69.37

24.00

6.63

16.00

10.50

ASIA

95.13

3.53

1.33

232.70

40.90

Middle East

15.93

2.49

1.36

19.79

80.53

12.58

6.89

24.70

40.60

Central Asia

9.19

0.14

0.02

9.35

98.29

1.54

0.16

13.20

28.50

Southern and Eastern Asia

174.17

4.77

1.41

180.35

96.57

2.64

0.78

194.80

42.20

OCEANIA

1.94

1.02

0.23

3.19

60.88

31.83

7.29

3.30

6.80

Source: FAO AQUASTAT.

The methods of surface irrigation are identified by the processes how water is applied, flows over to the soil surface, and infiltrates, as well as the shape and slope of the fields and the duration of the irrigation phases:

1. (i) Flooding methods—water is applied to flood the irrigated field, so the water is stored above the soil until infiltration is complete. These methods are the most ancient and continue to be the most used. They comprise paddy basins for rice cropping, where water ponding is maintained on the soil surface (Fig. 1a) or, in modern times, the soil water is just kept wetted, and non-rice basins, flat or furrowed (Fig. 1b), when they are flooded temporarily with a large time interval between successive irrigation events. The basin fields have very low or null slopes, are diked all around, and the surface may be flat or furrowed in case of row crops. Because the literature reports more often on borders and furrows than on basins, the latter are often called closed borders, while furrowed basins may be called furrow systems; however, distinctions are easy when observing the respective phases.

Figure 1. Flooding irrigation: (a) flooded rice paddies at early crop stages; (b) furrowed level-basin by the end of the ponding phase (photos by L. S. Pereira).

2. (ii) Infiltration methods—water is applied to gently sloping fields to favor infiltration while flowing to the downstream end, which is generally open, and non-infiltrated water flows into drainage ditches, from where it may be reused. The infiltration methods comprise border and furrow irrigation. Borders consist of sloping strips of land separated by border dikes that follow the contours when plantations follow the slopes (Fig. 2a). They have a flat soil surface where a longitudinal slope favors water slowly flowing over the field while infiltrating. In furrow irrigation, water is applied to small longitudinal channels which slope and inflow discharges are set to favor infiltration while water slowly flows to the downstream end (Fig. 2b). Both borders and furrows are commonly open at the downstream end.

Figure 2. Infiltration methods: (a) strip borders irrigating a vineyard, (b) furrow irrigation with supply with siphons placed in a water level controlled canal (photos by L. S. Pereira).

3. (iii) Surface runoff methods—water is spread from small upstream ditches and flows unconstrainedly over the land surface, generally pastures, and infiltrates the soil; the non-infiltrated water flows into downstream ditches or natural streams (Fig. 3). At present, these methods are more often used in pastoral mountain landscapes (Pôças et al., 2009, 2013).

Figure 3. Surface runoff irrigation (photo by I. Pôças).

4. (iv) Rainwater harvesting and spate irrigation—consisting, respectively, in using rainwater collected from upstream non-cropped land (Fig. 4a), or adopting land forms that favor runoff and its infiltration where the crop develops (Fig. 4b) (Oweis et al., 2012), and in diverting flood water from rivers or streams into the fields, with non-infiltrated water returning to the river or the stream network (van Steenbergen et al., 2010). Both methods are aimed at favoring water collection when rain occurs in dry areas, the first at a small scale of application, the second at larger areas where land is divided into basins.

Figure 4. Water harvesting: (a) sequence of “meskats,” in southern Tunisia, collecting runoff from the slopes and providing for its infiltration in small basins where palm trees and olive trees are cropped; (b) contour ridges, with harvested water infiltrating near the ridges where crops are planted (photos by L. S. Pereira).

# Performance Indicators

The performance indicators usually considered in SI studies consist of the distribution uniformity (DU, %) and the application efficiency of the low quarter (ea, %), the latter characterizing the beneficial water use fraction (Pereira et al., 2012). Following the approaches of Burt et al. (1997), and Pereira and Trout (1999), they are defined as follows:

$Display mathematics$
(1)

where Zlq is the average low quarter depth of water infiltrated (mm) and Zavg is the average depth of water infiltrated in the whole irrigated field (mm). Two equations are used for ea to distinguish the cases of over-irrigation (Zlq>Zreq) and under-irrigation (Zlq<Zreq):

$Display mathematics$
(2)

where Zreq is the average depth (mm) required to refill the root zone in the quarter of the field having higher soil water deficit and D is the average water depth (mm) applied to the field. Zreq is estimated from measurements or simulations of the soil water content before the irrigation, which is used to estimate the soil water deficit in the root zone (SWD, mm). Zlq and Zavg are estimated from computing the depth of water infiltrated during the irrigation process. D equals the product of the cutoff time, tco, by the average inflow rate, Qin:

$Display mathematics$
(3)

D = Zavg in case of basin irrigation, where all applied water infiltrates because full basin diking impedes drainage runoff to occur, and D > Zavg in case of furrows and borders open at the downstream end, therefore where drainage runoff occurs. Observing (1) and (2), it may be noted that ea cannot exceed DU. Their potential values are attained when Zlq approaches Zavg and this one equals Zreq, so DU is at its potential when the field characteristics, particularly length, slope, inflow rate, and surface leveling, provide for optimal and quasi-uniform infiltration. DU can be functionally described by:

$Display mathematics$
(4)

where Qin is the inflow rate (per unit width of the basin or border or per furrow), L is the length of irrigated field, n is the hydraulic roughness, So is the longitudinal slope of the field, Ic represents the intake characteristics of the soil, Fa represents the cross-sectional characteristics of the furrow, border, or basin, and tco is the cutoff time. The water application efficiency can be described by the factors of (4) together with the soil water deficit (SWD) when irrigation starts:

$Display mathematics$
(5)

DU primarily depends upon the adequateness of the designed basin, border or furrow—L, n, So, Ic, Fa in agreement with selected Qin and tco—to achieve an optimal combination of flow and infiltration into the irrigated field. Differently, the efficiency ea largely depends upon the appropriateness of the irrigation scheduling decision, that is, the irrigation timing and depth relative to the actual SWD. Therefore, DU is an indicator of the SI system performance, however influenced by the actual decisions on Qin and tco, while ea is an indicator of the adequateness of the irrigation scheduling applied (Pereira & Trout, 1999). In other words, the efficiency ea does not characterize the SI system but its management, which depends on both farmer decisions and constraints of the supply and delivery management (Pereira et al., 2012).

Improving the performance of SI systems requires a variety of measures and practices which provide for reducing water applied and higher land and water productivity but require appropriate combination with irrigation scheduling (Pereira et al., 2002, 2007).

The yield productivity per unit of land surface (Ya, kg ha−1) and water productivity (WP, kg m−3), often called water use efficiency, are also used as performance indicators. WP is the ratio of Ya to the water used (WU, m3) to achieve that yield. WU may refer to the total water used, the irrigation water, or the consumptive use, evapotranspiration. Both Ya and WP mainly depend upon crop and irrigation management, but they are also dependent on DU, since a low DU indicates that a large fraction of the field is under-irrigated while the other fraction is over-irrigated, which necessarily affects crop growth and yield.

# Irrigation Phases and Related Performance Improvements

The irrigation phases and respective time durations are essential to characterize the irrigation methods (Pereira & Trout, 1999). Irrigation phases (Figs. 5 and 6) differ among irrigation methods because water flow and infiltration processes are diverse. Phases are the following:

1. (i) Advance, characterized by the advance time (tadv), the time after irrigation starts until the wetting front reaches the downstream end of the field; during this phase, the process of water flow over the soil surface is dominant over infiltration.

2. (ii) Ponding or wetting, characterized by the ponding time (tpond), the time after advance is completed until cutoff, when water application ends (cutoff time, tco), thus tpond=tco−tadv. During this phase, water application aims to complete storage over the field or to provide for maintaining infiltration.

3. (iii) Depletion, essentially refers to the infiltration process and its time duration (tdep) develops from cutoff until water starts to disappear from the soil surface due to infiltration and runoff drainage.

4. (iv) Recession, where infiltration progressively ends over the field, has a time duration (trec) that develops after depletion until all water disappear from the soil surface and infiltration is completed.

Figure 5. Irrigation phases of a modern level basin (adapted from Pereira & Trout, 1999).

Searching for improved DU implies making infiltration as uniform as possible, thus equalizing the time opportunity for infiltration (τ‎) throughout the field (Fig. 5). This is achieved differently for the various SI methods.

In basin irrigation, it is essential to reduce the advance time (Fig. 5), which requires precise land-leveling, large but non-erosive Qin (1 to 5 l s−1 m−1), and not very large basin lengths (up to 100 m in general). A very small slope also reduces tadv and is important when land-leveling is less precise. The ponding time may be short or long depending upon whether the irrigation depth applied is small or large. The depletion time varies with the intake rate of the soil and with the amount of ponded water. Finally, the recession time is short, depending on the uniformity of the surface micro-topography. Despite τ‎ uniformity increasing with tpond and tdep, it is more dependable to shorten tadv as proposed above.

Water velocity in furrows has to be small to favor infiltration when water is flowing. Therefore (Fig. 6), tadv tends to be large because Qin and slope So have to be small, nevertheless depending upon the soil intake rate and the furrow length (commonly 200 to 400 m). The wetting phase is large because Qin is small (0.3 to 1.5 l s−1 per furrow), thus requiring a large tco for the application of the irrigation depth D. Unlike for basins, the depletion phase is quite short, since water normally starts to disappear from the furrow bottom shortly after cutoff. Finally, the recession phase is also short depending on Ic and L. DU is therefore improved by shortening tadv and lengthening the wetting phase, so using large D or reducing Qin when the furrows intake rate reduces after progressive sealing.

Figure 6. Irrigation phases of a furrow irrigated field (adapted from Pereira & Trout, 1999).

The advance in borders is quite long, because So and Qin have to be small aiming at water infiltrates while advancing over the field. Consequently, for long border strips it often happens that tadv is larger than tco; then, the wetting phase does not exist. It results that the depletion phase is somewhat large, as well as the recession phase, naturally depending upon the soil intake rate, L and So. It is therefore difficult to attain a short advance in borders, which is constrained by Qin, So, L, and intake rate, thus making it difficult to achieve high DU in borders except when So is very small.

As mentioned before, the efficiency ea highly depends upon DU in addition to irrigation management and scheduling. Therefore, high ea is achieved when DU is high and management is such that the applied water (Qin and tco) fits SWD at time of irrigating.

Performance indicators are not considered in surface water spreading over pastures and grasslands, since these systems, in addition to irrigation, provide environmental benefices in terms of landscapes and soil and water conservation (Pôças et al., 2009, 2013). They are also not considered with rainwater harvesting and spate irrigation, because these systems correspond to ingenious approaches to better use water in dry areas just when rain occurs, not through controlled water application as for systems referred to before. However, yield and flood-water benefices are considered when assessing these systems (Tubeileh et al., 2009; Hamdi et al., 2016).

# Modernization for Improved Water, Labor, and Yield Performance

The modernization of SI systems developed in the second half of the 20th century, when irrigated areas were expanding and pressurized systems, sprinkler and micro-irrigation, developed. This happened together with the progressive development of modern agriculture, including mechanization. Innovations were dictated by the need for reducing labor requirements, decreasing irrigation costs, improving irrigation water use and water saving, and controlling environmental impacts of cropping practices and irrigation (Table 2). Lately, SI areas have slowly decreased due to the competition of pressurized systems, mechanized and often automated, thus with lower labor requirements and easier control of water use, and whose market is well established and continuously growing. Contrarily, the market of surface irrigation equipment and services is small and did not develop where SI is dominantly practiced in small farms and labor costs are low, thus propitiating the use of traditional SI. These systems are adopted worldwide, mainly in Asia, for both rice paddies and field crops, although they are being slowly improved through adopting more efficient equipment of water distribution and control and/or precise land-leveling practices.

Table 2. Measures and Practices for Improved Performance of Basins, Furrows, and Borders.

Techniques

Benefits

Applicability

Precision land leveling

– Less water to complete advance, improved distribution uniformity; favorable conditions for deficit irrigation and to control the leaching fraction

– Applies to basins, borders, and furrows and is particularly important for basin irrigation

Basin irrigation

– Higher discharges, reduced widths and/or shorter flow lengths

– Fast advance time, reduced volumes applied, easier application of deficit irrigation and control of the leaching fraction

– Easy; limitations relate to field size and geometry

– Furrowed basins for row crops

– Faster advance, improved emergence and rooting of the crops planted on the bed

– Easy to implement

– Higher basin dikes to catch storm rainfall

– Providing for full conjunctive use of irrigation and rainfall

– Easy to implement, mainly for paddy fields

– Maintaining low water depths in rice basins

– Lower seepage and percolation and better conditions to store any storm rainfall

– Limitations when land leveling is poor and/or delivery is infrequent

– Non-flooded paddies, i.e., maintaining the soil water near saturation

– Lower seepage, deep percolation, and evaporation losses; better conditions to store any storm rainfall

– Only for paddies in warm climates and when deliveries are frequent

Furrows and borders

– Irrigation with alternate furrows

– Reduced water application to the entire field; favored deep rooting of the crops

– Easy to apply

– Reuse of tail water runoff

– Avoidance of runoff losses, increased systems efficiency, and controlled quality of return flows

– Need for collective facilities in case of small farms

– Closed furrows and borders

– Avoiding runoff at the downstream end

– Needs controlled inflow discharges to avoid waterlogging downstream

– Contour furrows

– Runoff and erosion control in sloping land

– When fields are not oriented down slope

– Surge flow

– Faster advance, reduced percolation and runoff, higher performance; provides for system automation

– Easy for large farms but difficult for small ones. Needs level basins or slight slopes to avoid erosion

– Continuously decreased inflow rates (cablegation)

– Control of percolation and runoff by adjusting flow rates to infiltration, and provides for system automation

– Requires technological support and is difficult for small farmers

– Irrigation with anticipated cutoff

– Reduced water application; runoff is minimized and percolation is reduced

– When available water is limited; applies better to borders and furrows

On-farm water distribution

– Gated pipes and layflat pipes

– Easier control of discharges, control of seepage; provides for using automation valves (e.g., surge flow) and cablegation

– Easy to adopt but requires investment by farmers

– Buried pipes for basins and borders

– Easier control of discharges, avoidance of seepage, and favored conditions for automation (e.g., cablegation)

– Less appropriate for small farms

– Lined on-farm distribution canals

– Easier control of discharges when using siphons, canal gates, and weirs; avoidance of seepage

– Only for large farms

– Improved on-farm earth canals

– Easier control of discharges when using siphons; reduced seepage

– Limitations relative to farm implements available

– Automation and remote control of farm systems

– Improved conditions for operation; favored application of irrigation scheduling and precise real-time irrigation management

– Application only to large farms with high technological and financial capabilities

Source: Adapted from Pereira et al. (2009).

## Basin Irrigation

Basin irrigation modernization includes:

1. (i) Precise laser land-leveling, aimed at achieving high DU (Fangmeier et al., 1999; Bai et al., 2011).

2. (ii) Qin control equipment aimed at adjusting it to basin intake rate, L and So, thus to achieve short tadv and high DU. Equipment to control Qin includes: (a) automated or semi-automated canal check gates to regulate weirs or pipe water turnouts (Fig. 7a); (b) pipe outlets with gate control; (c) outlet boxes to derive water from earthen canals; (d) gated pipes to distribute water at the upstream end; (e) buried pipes equipped with valve-controlled risers; (f) buried pipes with risers and water control with a cable-plug system (Fig. 7b), buried cablegation (Trout & Kincaid, 1989).

Figure 7. Farm water supply to basins under operation: (a) canal equipped with automated check gates and side long crested weirs; (b) buried cablegation system with risers supplying basins (photos by L. S. Pereira).

3. (iii) Shortening the basins and/or reducing their width for short tadv and higher DU when Qin is small and/or variable.

4. (iv) Automatic control devices and sensors for automation of gates and valves.

5. (v) Furrowed basins for row crops that improve advance and DU and avoid excess wetting of plants.

Paddy rice basin irrigation typically consists of basins where ponding is continuous during the crop season and water applications are frequent, from daily to a few days. Precise land-leveling is commonly used in large farms, mainly in Europe and Americas. Water delivery to fields use the same type of water control devices referred above for non-rice basins. Various paddy rice irrigation methods exist:

1. (i) Conventional paddies, where ponding is deep, near 10 cm, and irrigation frequency varies from daily to a few days; this method is appropriate where water plays a main role in temperature regulation, and the present tendency is to adopt reduced depths, variable along the crop season, thus decreasing percolation and reducing irrigation water use.

2. (ii) Shallow-ponded water paddies, where ponding depths are close to 5 cm and daily irrigation events are common, resulting in less water use and high yields (Mao et al., 2004); border dikes are kept high in the monsoon areas to keep rainwater inside the basins.

3. (iii) Intermittent flooding, corresponding to aerobic rice, where climate favors natural soil drying; irrigation events have large intervals between successive ponding, and bordering dikes are high in the monsoon areas to collect rainwater; this approach aims at water saving and decreasing methane emissions.

4. (iv) Permanent soil wetting, with frequent irrigations but without ponding; this approach aims at water saving and high yields in warm monsoon climates.

5. (v) Floating rice paddies, where basins allow for deep water and rice varieties adapt to grow when floating in deep water; floating rice basins are used in areas with very high precipitation, as in the Mekong Delta, where they show good yield and economic results comparable to conventional paddies (Nguyen et al., 2015).

Research for modernizing paddies generally considers water saving, yields, reduction of greenhouse gases, and multi-functionality (Matsuno et al., 2006; Sharma et al., 2016).

## Infiltration Methods: Border Irrigation

To avoid erosion, the soil should be covered as happens to grasslands. In orchards and vineyards, the crops are planted on the border-dike and the inter-row space consists of the border strip (Fig. 2a), where active vegetation may grow. If borders are open at the downstream end, the non-infiltrated water drains into a downstream ditch and may be reused in other fields or pumped back to the same field. To control outflows, anticipated cutoff may be used (Niblak & Sanchez, 2008). Border irrigation is gradually declining in use and borders are being converted to basins to irrigate field crops, namely row crops adopting a ridge-furrow system, when land slopes are small, or they are converted to sprinkler irrigation. Orchards and vineyards border irrigation are quite often converted to micro-irrigation. Borders modernization, as for basins, consists of adopting precise land-leveling, equipment, and tools for Qin control (Morris et al., 2015), and shortening and/or reducing the width of the border strips for reducing tadv and improving DU, as well as adopting drainage reuse for water saving.

## Infiltration Methods: Furrow Irrigation

To improve the performance of furrows, thus achieving a short advance and high DU, in addition to precise land-leveling, various practices are used aimed at controlling furrows Qin, mainly considering that furrows intake rate decreases with time. The intake rate is higher when the furrows are dry, because soil macroporosity is then high; soil macropores tend to progressively seal after furrows wetting, due to sedimentation of small soil eroded particles carried by the flowing water. The control of Qin may be achieved with:

1. (i) Gated pipes, including lay-flat tubes, which supply water to each furrow and may be controlled with automated valves.

2. (ii) Surge-flow irrigation, generally using automated valves that provide for on and off cycles of water delivered to the gated pipes located on both sides of the valve (Fig. 8), so producing on and off cycles of inflow to the furrows (Humpherys, 1989). Commonly, four cycles are used during the advance phase. After the advance is completed, on and off cycles may continue or, more often, the initial Qin is halved and applied simultaneously to furrows on both sides of the valve. Impacts include improvements in DU, ea, water saving, and WP (Horst et al., 2007).

Figure 8. Surge-flow: furrows on left completed the first surge and those on right have just initiated that first cycle (photo by L. S. Pereira).

3. (iii) Irrigation with cutback, using automated valves or manually, directing half of initial Qin into other areas to reduce furrows inflow when their intake rate decreases, thus controlling drainage outflows (Campo-Bescós et al., 2015) and deep percolation, so improving DU.

4. (iv) Cablegation, which consists of an automated and continuous cutback through controlling the discharges of a gated pipe, placed on land with uniform slope, where a plug moves down at a controlled velocity (Fig. 9). The outlets located immediately upstream of the plug have essentially full pressure providing highest Qin, while further upstream pressure and Qin are reduced, that is, Qin decreases according to the velocity of the plug. Larger initial Qin favor a faster advance, and smaller Qin later help in controlling both drainage outflows and deep percolation (Kemper et al., 1987).

Figure 9. Cablegation, an automated cut-back control system for furrow irrigation.

5. (v) Earthen canals controlled with check gates to supply siphons feeding every furrow.

6. (vi) Farm water supply as referred to basins, namely: (a) canal outlets with pipe control boxes to derive water from earthen canals into farm earth ditches that supply furrows; (b) buried pipes with valve-controlled risers to supply the gated pipes; (c) related sensors and controllers.

## Rainwater Harvesting, Runoff Collecting and Spreading, and Spate Irrigation

Modernization of these systems essentially refers to the structures used to derive water from natural streams, to collect and apply harvested rainwater, and to derive and distribute flood water from rivers into the basins of the cropped area.

The SI sustainability depends upon its economic, social, water use, and environmental impacts, which are interlinked. Current research goes in these directions, but contradictorily. The lack of market of SI equipment and services and the lack of political will to support SI innovation contribute to a controversial discourse: “SI systems are not efficient, use too much water, and impact the environment through polluting surface and groundwater with nitrates and herbicides.” However, this discourse is not based upon innovation research, ignores the impacts due to external causes such as the referred lack of market and the impacts of poor fertilizer, crop and irrigation management, and that conditions for sustainable use of SI systems vary worldwide with climate, soils, environmental, agricultural, economic, social, and cultural contexts.

Sustainability of SI is impacted by a variety of factors. First, there is an insufficient adoption of equipment and tools for semi-automated and automated irrigation, particularly for Qin control. This is particularly important in countries where SI markets are poor or nonexistent and farmers have very limited access to SI equipment and services, contrary to what happens with pressurized systems markets. Lacking Qin control equipment, farmers face difficulties in using less water and better controlling contamination, the latter being particularly important when poor-quality water, wastewater, and saline water are used (Pereira et al., 2014). Contrarily, the implementation of water saving and contamination control with pressurized systems is easier due to availability of related equipment.

The poor use of performance terms, including by researchers, conveys a message that SI is inefficient. However, field observations of ea are often missing, and the dependence of ea to DU is generally not considered. Therefore, ea is used as the sole performance indicator without distinguishing the important impacts of irrigation and crop management in addition to DU. For China, Miao et al. (2015) reported that while DU of precise land-leveled basins was very high (up to 95%), ea was small (<60%) because farmers were not using an appropriate irrigation scheduling. The need for an approach assessing both DU and ea was reported by Pereira et al. (2002, 2012), including for controlling salinity.

There is evidence that well-designed and managed sprinkler and drip systems use less water than SI, particularly when the latter do not adopt precise land-leveling and/or improved water control devices (Darouich et al., 2014, 2017). The water consumption use of SI may be similar to pressurized systems, but percolation and runoff are greater. However, percolation is generally reusable as groundwater (Ahmad et al., 2014), while runoff may be reused or may add to streamflow downstream (Ahadi et al., 2013), that is, it does not consist of water losses but benefices to downstream users. Water-table levels influence groundwater contributions and salinity, particularly in SI systems. Thus, adopting efficient drainage that controls water-table levels contributes to the performance of SI systems, namely yields and WP (Xu et al., 2013).

It is well known that pressure control in pipes is easier then water level control in canals, thus conveyance and distribution in canal systems need to be modernized to allow delivery scheduling to match irrigation scheduling requirements; otherwise, varied discharges, volumes, and timings of deliveries impact the performance of SI systems (Gonçalves et al., 2007; Hashemy-Shahdany & Firoozfar, 2017).

There is evidence that percolation and runoff favor contamination of ground and surface waters, so that the risk of contamination is greater for SI relative to pressurized systems, mainly when land-leveling is poor and Qin is not properly controlled. This happens mainly in farms not adopting adequate irrigation scheduling, fertilizer management, or pesticide and herbicide practices (Azevedo et al., 2000; Causapé et al., 2006). NO3 leaching and N2O emissions may be high in SI systems, namely in rice paddies, but they are also high elsewhere if fertilizer management is poor (Quemada et al., 2013). However, those N issues are aggravated when land-leveling and Qin control are poor and irrigation scheduling is inadequate. Methane emissions from rice paddies are controllable using paddies without ponding, with intermittent ponding or seeding dry and later allowing soil drying (Kumar et al., 2016; Peyron et al., 2016).

Yields may be similar to those obtained with pressurized systems, but, as mentioned before, water productivity is generally smaller because SI uses more water. WP of SI systems, however, increases if deep percolation and runoff are controlled. Energy use in SI farms is reduced to farming operations and land-leveling, and is obviously much less than for pressurized systems, which rely on electrical or diesel energy to properly convey, distribute, and apply water. Production costs with SI are smaller than for pressurized systems because energy and investment costs in equipment are generally higher than costs of land-leveling. Darouich et al. (2014, 2017) reported that advantages of drip or sprinkler over SI systems occur when the priority is water saving but not if the priority is the economic return to the farmer.

In conclusion, SI systems can be sustainable if control of inflows is practiced, precise land-leveling is adopted, systems are properly designed, irrigation scheduling is appropriate, fertilizer management is adequate, and crop management, including the control of pests and weeds, is appropriate. These requirements imply farmers’ incentives and adequate extension services.

# Current State of the Science

## Processes Modeling

The process of surface irrigation combines the hydraulics of surface flow over the irrigated land or in the furrows with the infiltration of water into the soil profile. Surface flow is unsteady and varies spatially. The flow at a given section in the irrigated field changes with time depending upon the system geometry, resistance to flow, and soil infiltration, which also change in space and time. The equations describing the hydraulics of surface irrigation are the Saint-Venant continuity and momentum equations (Walker & Skogerboe, 1987). The continuity equation expresses the conservation of mass and can be written as

$Display mathematics$
(6)

where t is time (s), Q is the discharge (m3 s−1), x is the distance (m) from the upstream section, A is the flow cross-sectional area (m2), and I is the infiltration rate per unit length (m3 s−1 m−1). Several infiltration equations are used, most commonly the empirical Kostiakov equations:

$Display mathematics$
(7a)

$Display mathematics$
(7b)

where I is the infiltration rate per unit area [mm h−1], Z is the cumulative infiltration [mm], a and k are empirical parameters, τ‎ is the time of opportunity for infiltration [h], and f0 is the empirical final or steady state infiltration rate [mm h−1], commonly used in furrow infiltration where cutoff times are long and infiltration tends to approach a steady rate. When initial preferential flow occurs, as for swelling or cracking soils, an initial “instantaneous” infiltration amount must be considered. Other infiltration equations are sometimes used, namely the more precise two-dimensions Richards’ equation (Tabuada et al., 1995; Bautista et al., 2016). However, the complexity in parameterizing a model increases and precision of simulated infiltration is shadowed by the variability of soil hydraulic properties.

The momentum equation, expressing the dynamic equilibrium of the flow process, is

$Display mathematics$
(8)

where g is the gravitational acceleration (m s−2), So is the land (or furrow) slope (m m−1), Sf is the friction loss per unit length or friction slope (m m−1), v is the flow velocity (m s−1), and y is the flow depth (m). These equations are first-order non-linear partial differential equations without a known closed-form solution.

The current approaches to solutions of (6) and (8) are: (a) the method of characteristics, converting these equations into ordinary differential ones; (b) the Eulerian integration, based on the concept of a deforming control volume made of individual deforming cells, which provides for the complete solution of the Saint Venant equations; (c) the zero-inertia approach, assuming that the inertial and acceleration terms in the momentum equation (8) may be neglected in most cases of surface irrigation aiming at avoiding difficulties inherent to the full hydrodynamics solution; and (d) the kinematic-wave approach, which assumes that a unique relation exists describing the Q = f (y) relationship but that is not applicable to level basins or zero-level furrows. Advances for solving these equations resulted from mathematical computing developments and from accurate observations and field evaluation of irrigation systems, including, recently, from the use of modern sensors. Consolidated reviews and description of those solutions, using the Kostiakov infiltration (7), are given by Walker and Skogerboe (1987) and Strelkoff and Clemmens (2007).

The Eulerian approaches and related simplifications are incorporated in various mathematical models. Strelkoff and Katopodes (1977) first presented an application of zero-inertia modeling for border irrigation, and various modeling applications were then developed, as for furrow surge flow modeling (Oweis & Walker, 1990). The first computer programs for surface irrigation included BRDRFLW (Strelkoff, 1985), BASCAD (Boonstra & Jurriens, 1988), BASIN (Clemmens et al., 1993), BORDER (Strelkoff et al., 1996) and SURDEV (Jurriëns et al., 2001). Currently, the most commonly used models are SRFR (Strelkoff et al., 1998; Bautista et al., 2015) and SIRMOD (Walker, 2003).

The simple volume balance equation, which describes the continuity equation, can be appropriate for many practical problems of sloping furrows and borders (Walker & Skogerboe, 1987; Bautista et al., 2012). That equation expresses the time varied relationships between inflow discharge and the volumes infiltrated or stored on the surface:

$Display mathematics$
(9)

where Q0 is the inflow discharge (m3 min−1), t is the supply time (min), Vy (m3) is the volume of water stored on the soil surface, and Vz (m3) is the volume infiltrated. Modeling with (9) is particularly useful to study the advance phase and to derive infiltration characteristics from observations of advance (Smerdom et al., 1988; Gillies & Smith, 2005).

Field evaluation of farm irrigation systems plays a fundamental role in improving surface irrigation. The first consolidated guidelines are those by Merriam and Keller (1978). Evaluations detect system problems, produce information to advise irrigators on how to improve their systems operation, and provide data for improved design. Basic field evaluation includes observation of: (a) inflow and outflow discharges and volumes, following equipment developments by Bos (1976) and Replogle et al. (1990), (b) the irrigation phases, particularly advance and recession, (c) soil water storage, (d) field geometry and leveling conditions, (e) soil erosion (Trout, 1996; King et al., 2016), (f) hydraulic roughness estimations, and (g) applied irrigation management procedures. Aimed at model parameterization, various procedures have been developed (Katopodes et al., 1990).

Measuring the infiltration characteristics is essential for surface irrigation performance assessment and design, considering its spatial and temporal variability. Methods include the cylinder infiltrometer, ponding (e.g., basin infiltrometers), inflow-outflow field measurements, blocked furrow method, and the recycling furrow infiltrometer (Walker & Skogerboe, 1987). More recently, Kostiakov infiltration parameters are derived from applying the inverse modeling using advance, recession, and runoff measurements (McClymont & Smith, 1996; Gillies & Smith, 2005).

## Decision Tools for Design and Ranking

Traditionally, the design of surface irrigation systems has been based on the application of empirical rules. Advances in simulation modeling, computing, and user-friendly software for personal computers provided good tools for design of surface irrigation systems, as described by many authors (Walker & Skogerboe, 1987; Clemmens et al., 2007; Reddy, 2013). In addition to hydraulics simulation modeling, design requires the application of other approaches such as for land leveling (Dedrick et al., 2007), water delivery and distribution systems (Walker & Skogerboe, 1987; Humpherys, 1995; Replogle & Kruse, 2007), and cost and environmental analysis (Khan et al., 2010; Gonçalves et al., 2011). Moreover, design requires identification of irrigation scheduling options to be used (Pereira, 1999), when aiming at controlling salinity (Pereira et al., 2007), and the consideration of supply management to be practiced in the area (Pereira et al., 2002). Design software may then comprise the canal or pipe supply system, as for the models SEDAM (Gonçalves et al., 2007) and SIDES (Adamala et al., 2014).

The decision support systems (DSS) methodology makes handling data of various types easier and favors the integration of simulation models and their interactive application. A variety of DSS have been developed in the general irrigation domain, e.g., Rao et al. (2004) and Gonçalves et al. (2007), relative to canal management and assessing alternative delivery schedules; Thysen and Detlefsen (2006), aimed at advising farmers; Khan et al. (2010), focusing on irrigation infrastructure investments; and Anane et al. (2012), for ranking sites for irrigation with reclaimed water. Only a few DSS focus surface irrigation design (Gonçalves & Pereira, 2009; Flores et al., 2010); however, a DSS using multi-criteria analysis provides for comparing and ranking alternative design solutions following various objectives and criteria, such as achieving water saving, attaining high water productivity, or maximizing farm incomes, as described in the following two examples.

These examples refer to cotton irrigation in Ras-El-Ain area, located in the northeast of Syria, where the climate is semi-arid and the challenge for sustainable water use in agriculture is great (Darouich et al., 2012, 2014). The first study aimed at ranking various design solutions for graded furrows and graded borders considering precise land leveling (GF and GB) and non-precise land-leveling (GFNLL and GBNLL), which is less costly but provides for less good irrigation performance. The total available discharge was 40 l s-1, and various inflow discharges were considered. The reference condition was to the traditional zigzag furrowed basins system. All design solutions referred to a field 100 m long and 50 m wide, with 0.8% longitudinal slope, zero cross slope, and medium infiltration characteristics (Darouich et al., 2012).

Indicators of water use in Fig. 10 refer to: (a) the beneficial water use fraction (BWUF), ratio of beneficial water used to total irrigation water applied (TIW); (b) irrigation water productivity (IWP), ratio of yield to TIW; (c) total costs of irrigation (TC); and economic water productivity ratio (EWPR), that is, the ratio of the yield value to TC (Pereira et al., 2012).

Results show that: (i) BWUF increases when precise land-leveling is adopted and decreases for larger Qin, and IWP behaves similarly to BWUF, since higher BWUF corresponds to less water use; (ii) total costs increase when precise land-leveling is applied, thus making EWPR inversely decrease. Overall, results show that there is a contradiction when searching for improved water use or to better economic results; design objectives must be clearly stated, namely when adopting multi-criteria analysis. For this application to cotton, there is a slight advantage of furrow over border irrigation. This was also the preference of farmers.

Figure 10. Comparing beneficial water use fraction (a), irrigation water productivity (b), total cost (c), and economic water productivity ratio (d) for graded furrows and borders (GF and GB). The abbreviation NLL is used for non-precise land-leveling. Inflow rates (l s-1 furrow-1 or l s-1 m-1 in case of borders) are indicated in brackets (adapted from Darouich et al., 2012).

The second example compares drip and surface irrigation using similar indicators as above (Darouich et al., 2014). Drip irrigation design considered various alternatives: double and single row per lateral (DRL and SRL), emitter spacing (ES) of 0.5 and 0.7 m, various pipe layouts (L1 to L8), and diverse types of self-and non-compensating emitters (SC and NC). Surface irrigation design alternatives referred to graded furrows and borders, precise and non-precise land leveling (LL and NLL), and various Qin rates. Fields were 100 m or 200 m in length. Comparing surface and drip irrigation systems (Fig. 11), it was evidenced that: (i) total irrigation water use was smaller for drip and non-beneficial water use was much greater for SI, particularly for NLL systems, which means that drip provides for large savings in water; (ii) consequently, IWP was higher for drip systems; (iii) irrigation costs were much less in case of SI; and therefore (iv) EWPR was much favored in case of SI systems. Again, overall results show a contradictory issue, with a clear advantage of drip systems when searching for improved water use and saving, and, contrarily, the superiority of surface irrigation when searching for improved economic results. Clearly, these results make evident the need for considering economic results and not only water use indicators. Moreover, results show that there is a potential for further using surface irrigation, particularly when system improvements are adopted, namely land-leveling, control of inflow discharges, and adoption of appropriate irrigation schedules. Appropriate support to farmers is required for them to adopt innovative approaches that may provide for the sustainability of surface irrigation.

Figure 11. Comparing graded furrows and borders with drip irrigation relative to: (a) total irrigation and beneficial water use and irrigation water productivity, and (b) irrigation costs and economic water productivity ratio. The abbreviations LL and NLL are used for precise and non-precise land-leveling. Inflow rates (l s-1 furrow-1 or l s-1 m-1 in case of borders) are indicated in brackets. Abbreviations for drip systems are: SRL and DRL for single and double rows per lateral, ES for emitter spacing, L1 to L8 for layouts adopted, and SC and NC for self- and non-compensating emitters. Emitters discharges (l h-1) are also indicated (adapted from Darouich et al., 2014).

# Future Trends

Surface irrigation is slowly losing importance but will continue to be a more common irrigation method than pressurized systems, which require higher investment costs and operation energy. However, SI faces a great variety of challenges whose solutions are difficult. Related advances essentially refer to engineering and management, but progress is limited. Nevertheless, a potential exists to adopt equipment and software tools in SI practice that will help improve irrigation performances, reduce water use, attain higher yields, and control percolation and runoff operational losses, reducing surface- and groundwater pollution and contamination as well as controlling GHG emissions, namely from rice paddies.

Progress in automation and semi-automation of conveyance and supply systems, mainly relative to hydraulic head and discharge controllers for canals and low-pressure pipes, is evident worldwide and has made easier off-farm water management of collective systems. However, this is less true in traditional systems where rotational water deliveries are the rule, e.g., “warabandi” systems in the Indus basin, and where irrigated farms are small. This is an engineering domain where progress is required, particularly aimed at equity in deliveries to farms, but it faces difficulties relative to requiring appropriate interaction between delivery schedules and crop irrigation schedules whose actors, canal managers and farmers, have different and often opposed interests and priorities (Shah et al., 2016).

Despite progress in the automation of canal and pipe on-farm supply systems (Walker & Skogerboe, 1987; Replogle & Kruse, 2007), farmers often prefer hand operation, because automation may require sensors, controllers, and wireless transmission-reception devices, which are costly, quite demanding in terms of operation, and not available ready to use. Automated alfalfa valves may be an exception, due to their simplicity and availability in the market. Meanwhile, research is proposing new tools like those for paddies (Masseroni et al., 2017).

Research aimed at the automation of furrows, basins, and borders has been enormous (Walker & Skogerboe, 1987; Humpherys, 1989, 1995), but farmers have not adopted them as expected; for example, the adoption of cablegation systems (Kemper et al., 1987) has remained far behind expectations, likely due to complexity in management. Future trends are likely to involve adopting sensors for water advancing in furrows or borders and using wireless transmission and controllers equipped with receivers and specific operational algorithms (Koech et al., 2014; Arnold et al., 2015). However, the adoption of such tools by farmers implies that related equipment is industrialized, cheap, and easy to operate. In addition, the appropriate design of those tools implies an improved use of models such as SRFR and SIRMOD to support the optimization of inflow discharges, field sizes, and slopes.

Adopting Qin control, the anticipated cutoff in borders and cutback in furrows aimed at reducing percolation and runoff may be achieved with automation, such as cablegation or modified surge-flow. However, this requires focused modeling. Adopting real-time control of Qin and of the cutoff time using sensors and transmission devices, which requires special consideration of infiltration variability in time and space (Bautista et al., 2014; Nie et al., 2017), is now a current research subject. This precise SI approach is quite demanding, may require better knowledge and modeling of the coupled surface and subsurface flow processes (Furman, 2008), and is apparently difficult for farmers to adopt. An easier but rougher approach is to adopt soil intake families for design modeling (Walker et al., 2006), which is quite useful for improved design as well to support Qin control in areas where soil infiltration data is scarce.

Land leveling has revealed quite important for improved SI performance, particularly basins. However, the spatial variability of infiltration adds to that of micro-topography in affecting DU (Bai et al., 2017). Moreover, the avoidance of water percolation out of the root zone is not achievable only through precision land leveling but requires adopting appropriate irrigation schedules, namely Qin and tco, that fit the crops demand and the performance of SI systems. Exploring an irrigation scheduling model associated with a surface irrigation simulation model is likely required to support design and, mainly, improved operation of the systems (Miao et al., 2015). The adoption of precise land leveling by farmers is, however, limited when field sizes are small and the crops sequence reduces the time when land is uncropped and a leveling operation could be performed. Incentives, including of a financial nature, may then be required.

NO3 losses and N2O emissions may be controlled if appropriate fertilizer scheduling is adopted. The application of fertilizers with the irrigation water—fertigation—may be helpful in controlling fertilizer impacts (van der Gulik et al., 2007), although precise land leveling is also needed for higher DU. The control of impacts of other chemicals may also be achieved through controlled application and, likely less probable, if applied with irrigation water, chemigation.

The control of methane and GHG emissions from paddy fields is a challenging area. To achieve that control, paddy water management has to be modified, as previously discussed, by adopting aerobic rice and intermittent paddy wettings (Chang et al., 2016; de Maria et al., 2016; Kumar et al., 2016; Sharma et al., 2016).

Adopting appropriate soil tillage, in particular conservation tillage, favors furrow firming in more permeable soils and improves soil aggregation in low intake soils, and thus may lead to improved soil and water conservation, reduced operational water losses, and better crop development and yield (Ahmad et al., 2014). This may be greatly relevant when irrigating in saline environments (Hoffman & Shalhevet, 2007).

In addition to performance evaluation surveys, and hopefully in combination with them, there is the need to perform focused economic surveys at the farm level aimed at assessing the economics of water use and saving and related water productivity. That information is required to better parameterize models and DSS usable for design and operation management of SI. In combination with the former, it is also required to develop easy-to-apply methodologies to monitor environmental impacts of irrigation, particularly greenhouse gas emissions, transport of solutes to ground- and surface waters, and salinity impacts. Specific survey focusing public health issues are required when reclaimed wastewater is used.

The developments considered above mostly involve large and/or commercial farms in countries such as the United States and Australia, where good markets exist for equipment, software, and consulting services. Thus, areas requiring innovation include small farms, mainly in areas where the equipment market is limited or nonexistent, which are dominant worldwide. Farmers likely face economic and managerial difficulties to adopt existing innovation tools. Limited issues are yet considered. Developments are also required in institutional terms that favor participation of farmers in water resources management, provide for incentives to farmers, and support extension services, including irrigation scheduling.

# Conclusions

Surface irrigation systems have been used for millennia and continue to be the most important irrigation system worldwide. There is a large variety of surface irrigation methods, revealing their adaptability to climate, crops, land forms, and cropping techniques. Their sustainability is evident in economic, productive, and social terms. Likely, surface irrigation methods and their wide adaptability to a range of climatic and socioeconomic contexts will contribute to the resilience of agricultural systems to global change, mainly considering modernization issues involving environmental sustainability of irrigation. The very low energy demand and the low investment requirements also contribute to irrigation sustainability. However, the challenges to be faced are enormous.

Surface irrigation has known great changes where new technologies and management tools are adopted. However, those tools are difficult to access for small farms and areas where a specialized market does not exist, farmers have little access to investment facilities, and the incentives and extension by adequate institutions are lacking. These farming conditions represent the majority of cases, but information and accessibility to advanced equipment and models use are also limited in the case of large and commercial farms. Without proper institutional arrangements, the use of pressurized systems is favored.

The sustainability of surface irrigation systems does not depend only on irrigation advances but relates to agricultural production systems. On the one hand, a successful irrigation not only depends upon the adequacy of water application technologies but also upon irrigation scheduling and, for collective canal systems, of management rules and delivery scheduling, which should match crop and farming requirements. An appropriate management of irrigation greatly limits operational losses of water by deep percolation and runoff. On the other hand, the appropriate control of fertilizer, herbicide, and pesticide application reduces pollution and contamination of surface- and groundwater and, at same time, provides for improved yields and economic results of irrigated crops. This is particularly important in saline environments and when wastewaters are used; then, a proper management of irrigation not only favors the environment but also reduces risks that may affect human health.

The need to access hydrologic impacts of surface irrigation at the basin scale is great. On the one hand, such assessment may lead to better understand the relationships of surface irrigation and groundwater, both in terms of water fluxes and of water quality. In fact, the commonly mentioned inefficiencies of SI may well favor groundwater recharge, and water quality impacts may be reduced if proper practices of fertilizer and chemical use are adopted. On the other hand, inefficiencies of upstream SI systems may also favor the availability of surface water to irrigators downstream. While the last decades of the 20th century were successful in understanding processes at field level and promoting innovations usable at the farm, a priority for this century may be to enlarge the scope of SI research to better understand processes occurring at the basin scale and to promote the farm innovations that, in addition to the farm scale, have positive impacts at a basin scale. Naturally, remembering the propositions by the Nobel Prize laureate Elinor Ostrom (1992), approaches must consider the need for reinventing institutions for self-governing irrigation systems that should support farmers in adopting more environmentally friendly practices which would contribute to the sustainability of surface irrigation.

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