Water savings and conservation in irrigated agriculture

8.8.1. Demand management: general aspects

Demand management for irrigation to cope with water scarcity consists of reducing crop irrigation requirements, adopting irrigation practices that lead to higher irrigation performances and water saving, controlling system water losses, and increasing yields and income per unit of water used. It includes practices and management decisions of an agronomic, economic, and technical nature.

The objectives of irrigation demand management can be summarised as follows:

ƒ Reduced water demand through selection of low demand crop varieties or crop patterns, and adopting deficit irrigation, i.e. deliberately allowing crop stress due to under-irrigation, which is essentially an agronomic and economic decision.

ƒ Water saving / conservation, mainly by improving the irrigation systems, particularly the uniformity of water distribution and the application efficiency, reuse of water spills and runoff return flows, controlling evaporation from soil, and adopting soil management practices appropriate for augmenting the soil water reserve, which are technical considerations.

ƒ Higher yields per unit of water, which requires adopting best farming practices, i.e.

practices well adapted to the prevailing environmental conditions, and avoiding crop stress at critical periods. These improvements result from a combination of agronomic and irrigation practices.

ƒ Higher farmer income, which implies to farm for high quality products, and to select cash crops. This improvement is related mainly to economic decisions.

BOX 8.2. Case study on soil management for improved water conservation

Experiments at two locations in the North China Plain, Daxing and Xiongxian, were developed to identify soil management issues for soil water conservation. The annual rainfall varies from near 330 mm to 700 mm. Rainfall is concentrated in July and August while during the winter wheat season it is only 17% of the annual total. The soils are very silty at both sites, containing more than 70% of silty particles, and have a poor non-stable structure. Salinity effects are negligible but soils have high pH. Soils have limited soil fertility and high exchangeable sodium percentage (ESP). Several soil treatments were compared and various soil properties and crop performance results were analysed (Ding and Hahn, 1998).

1. Bulk density/soil compaction: Subsoiling, i.e. subsurface tillage for soil loosening, significantly reduced the bulk density in the 20-40 cm layers but effects are reduced by the third year due to recompaction. Gypsum application also favoured lower bulk density.

2. Soil aggregate stability: Gypsum significantly increased aggregate stability.

3. Soil chemical properties: Soil pH decreased after gypsum application. The cation exchange capacity was greatly increased indicating better soil nutritional conditions. Exchangeable sodium percentage decreased very significantly, which favoured improved soil permeability and structural stability.

4. Infiltration rate: Subsoiling gave a consistently higher infiltration rate than the commonly practised disk harrow tillage and no tillage by breaking up the compacted plough pan.

Gypsum addition increased the infiltration further throughout the season due to stabilising the soil structure.

5. Water storage: The soil water storage in the mulch treatment was higher than that in other treatments, mainly when the rainfall was lower than the average. Early in the season, it was observed that the soil surface under mulch remained moist for 10 days after rainfall, whilst in other areas, it became dry in 3 to 4 days. The cause is that straw mulch reduces evaporation from the soil. The no-tillage showed higher water content in the top layer (0-30 cm) three weeks after maize sowing.

6. Crop emergence: Mulch produced the highest emergence rates because it favoured the topsoil retaining moisture and controlled the formation of crusts on the soil surface.

7. Summer maize roots: Subsoiling tillage significantly increased root depths, length and mass, which improves the use of water stored in the soil.

8. Summer maize yield: Mulch produced the highest yield followed by subsoiling. For the soil with higher silt percentage, the best yields were obtained by associating subsoiling with mulch. These results indicate that changing the behaviour of the soil relative to water infiltration, storage and evaporation induces yield impacts in line with improvements in soil water availability.

The agronomic aspects of irrigation demand management refer essentially to those described in the previous chapter. They concern crop improvement relative to resistance to water stress and respective water productivity, cropping techniques that favour coping with lesser water availability, and soil management for water conservation. Economic decisions, not dealt with here, concern the decision making processes relative to the selection of crop patterns and farming practices that reduce the crop irrigation demand, and include the evaluation of the economic returns and feasibility of water saving and conservation practices.

The technical aspects of demand management which concern the various practices within irrigation are dealt in this section.

Issues for irrigation demand management often refer only to irrigation scheduling, giving to irrigation methods a minor role. However, an integrated approach is required (Pereira, 1996; 1999). Irrigation scheduling is the farmers decision process relative to “when”

to irrigate and “how much” water to apply at each irrigation. The irrigation method concerns

“how” that desired water depth is applied to the field. The crop growth phase, its sensitivity to water stress, the climatic demand by the atmosphere, and the water availability in the soil determine when to apply an irrigation or, in other words, the frequency of irrigation.

However, this frequency depends upon the irrigation method, i.e. on the water depths that are typically associated with the on-farm irrigation system. Therefore, both the irrigation method and the irrigation scheduling are inter-related.

Irrigation scheduling requires knowledge of crop water requirements and yield responses to water (cf. Allen et al., 1998), the constraints specific to the irrigation method and respective on-farm delivery systems (cf. Pereira and Trout, 1999), the limitations of the water supply system relative to the delivery schedules applied, and the financial and economic implications of the irrigation practice. To improve the irrigation method requires the consideration of the factors influencing the hydraulic processes, the water infiltration into the soil, and the uniformity of water application to the entire field. Therefore, irrigation demand management to cope with water scarcity is discussed here with respect to both the irrigation systems and scheduling.

Several performance indicators are currently used in on-farm irrigation. The uniformity of water application to the entire field is commonly evaluated through the distribution uniformity (DU), which is the ratio between the average infiltrated water depth (mm) in the low quarter of the field and the average infiltrated water depth (mm) in the entire field (Burt et al., 1997; Pereira, 1999). The distribution uniformity essentially depends upon the characteristics of the irrigation system and less on the farmer management. In other words, high DU can only be achieved when the farmers manage the irrigation system well and it is well designed and maintained, whilst poorly designed and/or maintained irrigation systems definitely lead to low DU (Pereira et al., 2002).

The main farm efficiency indicator is the application efficiency (AE), the ratio between the average water depth (mm) added to root zone storage and the average depth (mm) of water applied to the field. AE is a measure of the quality of irrigation management by the farmer and is strongly related to the appropriateness of decisions on when and how much water is applied. Due to the limitations imposed by the system characteristics, the application efficiency depends upon the distribution uniformity. In general, when the distribution uniformity values observed are high, the application efficiencies will also be good when irrigation scheduling is appropriate.

Useful relations between irrigation uniformity and crop yields have been made available and may be helpful to practitioners (e.g. Warrick and Yates, 1987). These relationships show that attaining high DU is a pre-condition to achieve high application efficiencies, and therefore to obtain a good match between the amounts of water applied and the crop use requirements. Therefore, DU is an indicator that relates well to the system characteristics that favour water conservation and saving, as well as to higher water productivity. Therefore improvements in performance of farm irrigation methods and systems will be mainly discussed relative to their effects on distribution uniformity.

8.8.2. Demand management: improving surface irrigation systems Several surface irrigation methods are used in practice. The main ones are:

ƒ Basin irrigation, which is the most commonly used irrigation system world-wide. Basin irrigation consists of applying water to levelled fields bounded by dikes, called basins.

Two different types are considered, one for paddy rice irrigation, where ponded water is maintained during the crop season, and the other for other field crops, where the ponding time is short, just until the applied volume infiltrates. For non-rice crops, basin irrigation can be divided into two categories: traditional basins, with small size and traditional levelling; and modern precision-levelled basins, which are laser levelled and have large sizes and regular shapes. Especially with traditional basins, shape depends on the land slope and may be rectangular in flat areas and follow natural land contours in steep areas. For row crops, and especially horticultural crops, the basins are often furrowed with the crops being planted on raised beds or ridges. For cereals and pastures, the land is commonly flat inside the basin. Tree crops sometimes have raised beds around the tree trunks for disease control. Basin irrigation is most practical when soil infiltration rates are moderate to low and soil water holding capacity is high so large irrigations can be given. Basin irrigation depths usually exceed 50 mm. Inflow rates for basin irrigation have to be relatively high (> 2 l s^-1per meter width) to achieve quick flooding of the basin and therefore provide for uniform time of opportunity for infiltration along the basin length. Basins must be precisely levelled for uniform water distribution, because basin topography determines the recession of the ponded water.

ƒ Furrow irrigation: water is applied to small and regular channels, called furrows, which serve firstly to direct the water across the field and secondly act as the surface through which infiltration occurs. There is a small discharge in each furrow to favour water infiltration while the water advances down the field. Furrow irrigation is primarily used for row crops. Fields must have a mild slope, and inflow discharges must be such that advance is not too fast, nor too slow, i.e. the time elapsed since inflow starts at the upstream end until the water arrives to the other end must be in equilibrium with the infiltration to avoid either excess runoff at the downstream end, or excess infiltration in the upstream zone. Efficient furrow irrigation nearly always requires irrigation times longer than advance times. Runoff at the downstream end typically varies from 10 to 40% of the applied water, which should be collected, stored, and reused. Irrigation furrows are usually directed along the predominant slope of the field. Furrows are used on slopes varying from 0.001 m m^-1 to 0.05 m m^-1. Low slopes require soils with low infiltration rates. Slopes greater than 0.01 m m^-1 usually result in soil erosion.

ƒ Border irrigation: water is applied to short or long strips of land, diked on both sides and open at the downstream end. Water is applied at the upstream end and moves as a sheet down the border. Border irrigation is used primarily for close growing crops such as small grains, pastures, and fodder crops, and for orchards and vineyards. The method is best adapted to areas with low slopes, moderate soil infiltration rates, and large water supply rates. Borders are most common and practical on slopes less than 0.005 m m^-1 but they can be used on steeper slopes if infiltration is moderately high and the crops are close growing. Irrigation to establish new crops on steep borders is difficult because water flows quickly, is difficult to spread evenly, and may cause erosion. Design and management of very flat borders approximates conditions for level basins. Precise land levelling is required, and inflow rates should be neither erosive, nor producing too slow