Techniques of Rainwater Harvesting in Arid Regions | Water Management

The following points highlight the techniques employed for rainwater harvesting in arid regions. The techniques are: 1. On-Farm Reservoir/Farm Pond 2. Micro-Catchment Water Harvesting 3. Saucer Basin/Semi-Circular Bunds.

Technique # 1. On-Farm Reservoir/Farm Pond:

On-farm reservoir is simply a dugout excavation, which is easiest to construct for the purpose of storing water essentially from surface runoff. The on-farm reservoir or farm pond is the cost- effective method of the rainwater harvesting systems. An on-farm reservoir is constructed at a suitable place in farm where the runoff water flowing from entire farm should come and stored according to slope conditions. Shape of the on-farm reservoir may be square or rectangular and size may depend upon the catchment where runoff water is generated.

If slope of the farm land is not in a particular direction the land levelling is needed to make the land uniformly sloping towards a particular direction where reservoir can be constructed. A reservoir can be made small or large in size by varying its dimensions, i.e. length, width and depth.

While increasing size of the reservoir, attempt should be made to increase depth in greater proportion compared to other dimensions. This will keep exposure of surface water level in reservoir to sunlight at the least, which will result in relatively less evaporation ultimately leading to reduced loss of stored water. Besides, the reservoir would occupy relatively less land area, which may suitably be used for crop cultivation. Size of the on-farm reservoir should be decided after computing water supplies and demands.

The optimum size would be the minimum of available water quantities and required water amounts to irrigate certain area of land under particular crops. In general, about 10-15% of the farm land area can be utilized for constructing on-farm reservoir. The harvested rainwater can be subsequently used for providing supplemental or life-saving irrigation to winter season crops particularly in the condition of dry spells.

Thus, on-farm reservoir plays a significant role in conserving and managing surface water resources for the sustainable crop production in an arid climate. Machiwal et al. (2013) reported a success story of 12-year old on-farm reservoir constructed with 20000 m³ water storage capacity in arid region of Gujarat, India, which successfully supplied water for both drinking as well as irrigation purposes.

Technique # 2. Micro-Catchment Water Harvesting:

Micro-Catchment Water Harvesting (MCWH) is defined as a method of collecting surface runoff from a contributing area over a flow distance of less than 100 m and storing it for consumptive use in the root zone of an adjacent infiltration basin.

The MCWH technique requires development of small structures across mild land slopes, which capture overland flow and store it in soil profile for subsequent plant uses. Water availability to plants depends on the micro-catchment runoff yield and water storage capacity of both the plant basin and the soil profile in the plant root zone. The main idea behind the MCWH is that water collected and stored in the soil during relatively wet period can be used during dry period.

In arid environments of Gujarat, low rainfall, water scarcity and land degradation severely intimidate the production capacities of the rangelands. Surface crusting by rain drops generates frequent local runoff on degraded sloping land, but it is generally lost in transmission and seldom flows down to streams. Many studies such as Martinez-Mena et al. (1998), Bull et al. (2000) and Bergkamp (1998) support this phenomenon. Micro-catchments can capture this local runoff, reduce transmission losses and concentrate it into the plant basins.

The MCWH technique can be used to capture rainwater to improve soil-moisture and vegetation. After transplantation, rainwater harvesting can be used to speed up tree/crop establishment, deep root development and to reduce mortality rate. Water harvesting could increase crop productivity and diversity, decrease soil erosion and rehabilitate degraded lands.

A higher runoff efficiency of micro-catchments can capture a large proportion of the rainfall as run-on and concentrate this additional water for establishment of crops/trees. Deciding the micro-catchment area to cultivated area is of paramount importance. Micro-catchment area is a function of active root depth, soil water holding capacity of the soil, runoff coefficient, cultivated area, rainfall and actual evapotranspiration.

This technique of water harvesting has taken considerable attention of the researchers from the arid and semi-arid regions. Ali et al. (2010) assessed potential of the MCWH in a Mediterranean arid environment by using runoff micro-catchment and soil water balance approaches.

Technique # 3. Saucer Basin/Semi-Circular Bunds:

Semi-circular bunds or saucer basins are generally constructed as a network of earthen bunds shaped as half-circles with the tips facing upslope and on the contour. These bunds are very useful for raising trees and fodder, and this water harvesting system also improves range productivity. Size of the semi-circular bunds depends on the type of crop, soil and the amount of rainfall. In general, these structures are constructed in moderate to low rainfall areas (annual rainfall ranging from 200-750 mm), deep soils and low slopes.

The topography for constructing the semi-circular bunds should be even and level. The space in the form of gallery between tips of consecutive bunds is used for discharge of excess runoff. The top width of the bund is usually 10 cm and the height may be uniform where the topography is flat. The side slopes are 1:1 although flatter sides are also used. As the slope increases, the height is increased accordingly from the tip to the lowest point. The minimum height at the tip is 0.1 m.