11.1 Infiltration rates of different soils

Planning of farm ponds

Farm ponds are developed in different types of catchments. The nature, size, and slope of the catchments differ, and different types of farm ponds are possible. Three factors must be considered in the siting of farm ponds: soil type, topography, and catchment size.

Soil Types The soils associated with construction of a roadside farm pond should preferably have low hydraulic conductivity with minimum seepage and percolation so that water can be retained in the pond for long periods. Soils with low infiltration rates are most suitable for pond construction. Table 11.1 shows the infiltration rate of different soils. Clay and clay-loam soils have good potential for rainwater harvesting in unlined ponds, and ponds constructed of these soils suffer minimal seepage losses. Runoff water is naturally captured in ponds and depressions in such areas. Soils for pond construction should meet the following criteria:

• Soils at the pond site must be sufficiently impervious at the pond bottom. • Soils used to construct a pond must be compactable. Gravelly soils, sandy soils, or soils with certain clays are not suitable. • The best soils are sandy clay, sandy clay loam, or clay loam.

When geotextile liners are considered, standardizing the dimensions of the ponds is useful so that the geotextile lining can be prepared in advance rather than being made on site, which is more laborious and cumbersome.

Soils having outcrops and stones must be avoided when digging farm ponds. The soil profile depth should ideally be investigated before digging the pond to avoid wasting significant effort. Good soils extending to a depth greater than 1 meter that are free of stones and have low pH, low electrical conductivity, and low groundwater level should be selected for the farm-pond site. other soils may introduce difficulties. Peat soils have special problems because they are usually very acidic and need copious liming to yield usable water. Soils rich in limestone create problems of precipitating phosphate and iron.

Soil depth is also an important factor in the development of farm ponds. deep soils have the capacity to store harvested water for a longer duration. Soils measuring more than 1 meter deep are ideal for the construction of farm ponds. The greater the soil depth, the greater the depth of the farm pond: a deeper pond will reduce evaporation losses. A depth of 2.5 meters is often recommended for farm ponds to ensure an adequate volume of storage, low evaporation, and ease of access.

TABLE 11.1 Infiltration rates of different soils

SOIL TYPE

Sand

Sandy loam Silty loam Clay loam Clay

INFILTRATION RATE (MILLIMETERS/HOUR)

> 30

30–20

20–10

10–5

< 5

Topography Topography is an important consideration in planning farm ponds because it can affect the following:

• The size, shape, and depth of a pond • dam or pond embankment height • The speed and intensity of runoff into the pond • The simplicity or complexity of pond design and construction and thus cost • Safety issues

The topographic features of the farm catchment vary from place to place. In general, the land proposed for pond construction must accommodate a minimum amount of excavation to achieve an economical storage capacity. A narrow, deep pond will have a much smaller evaporation loss than a broad, shallow reservoir. If the land has some slope, the pond does not need to be excavated; a “u”-shaped bund can be used to create the pond. The road body itself can be part of the pond wall in some cases.

Drainage or Catchment Area The drainage or catchment area that produces runoff for farm-pond storage plays an important role in farm-pond planning. Road construction has a major impact on a catchment’s drainage, given that the road infrastructure often reshapes the catchment. Roads generally tend to combine smaller subcatchments and bundle runoff into a limited number of drainage canals, which effectively enlarges the source area of the pond. draining excessively large source areas into large ponds should be avoided, particularly if rainfall is concentrated in a short period (for example, if 80 percent of annual rainfall occurs in less than two months). These large ponds will require larger levees and have greater construction costs. More important, oversized source areas increase the risk of washouts and flushing; additionally, large ponds may require spillways and other water-control structures and are difficult to manage. A lower-cost alternative may be to connect a cascade of ponds, where water overflows from one storage pond into the next. If rainfall and runoff are more evenly distributed over the year, ponds can be smaller.

The pond must be filled at least once in the season so that farmers can use the water for critical irrigation and other applications during dry spells. depending on the rainfall pattern, a pond may be filled several times a year, increasing its effectiveness as a water-storage facility. oversized ponds and ponds serving small catchment areas may not fill adequately and may not offer adequate water supply during extended periods of hot, dry weather.

See box 11.2 for a summary of pond site requirements.

Design and layout of roadside farm ponds

The main factors in designing a roadside farm pond are the capacity of the pond based on water demand and rainfall patterns, and the pond’s layout, including shape, side slopes, and features such as silt traps and spillways.

Pond Capacity The design capacity of a roadside farm pond depends on the purpose for which water is needed and the amount of inflow that can be expected in a given period.

BOX 11.2

Siting of a roadside farm pond

The following factors should be taken into account when siting farm ponds.

• Selection of the farm-pond site depends on local soil conditions, area topography, drainage capacity, infiltration, and rainfall pattern and distribution. • Identification of natural depressions where rainwater or runoff either flows or accumulates during the rainy season may aid planning. • A good pond site will have level topography that provides for economical construction, soil with enough clay to retain the water, and an adequate water supply. • deep clay soils are best for lining ponds because they minimize leakage. Because a pond is simply a depression for holding water, the sides and bottom must be composed of a soil that minimizes seepage. • Coarse-textured sandy soils should be avoided because they are highly permeable, and water will drain through them. If seepage is believed to be high, pond walls can be plastered with clayey soil and compacted or lined with plastic. • Sites with underlying strata of sand, gravel, limestone, shale, or fractured rock at a shallow depth may also result in high leakage and seepage losses and should be avoided unless they are sealed with clayey soil. Peat soils have special problems because they are usually highly acidic and need substantial liming. • Farm ponds should be located where the area topography creates a road catchment from which sufficient runoff can be collected and redirected toward the farm pond by gravity. Whether the ponds can be supplied with water must be verified: the road embankment guides water either from road drains or culverts or from the runoff in drainage cuts. Water may be collected from the bends, low points, water bars, and rolling dips of unpaved roads. • For more solid construction and greater compaction of embankments, soils having a wide range of grain sizes are preferable to soils with relatively uniform particle size. • The choice of pond sites should balance economics, accessibility, and safety. A pond that provides the largest volume of water with the least amount of landfill offers the most economical construction.

Liability is also an important consideration in pond site selection. The potential impacts of pond or dam failure should be assessed carefully, especially if failure could cause loss of life or injury. • Siting should ensure that polluted or contaminated water is not harvested into farm ponds. • Provision must be made for a pipe and emergency spillway, if necessary. The runoff flow patterns from these structures must be considered when locating the pond or pit and placing the spoil.

Source: Based on Nissen-Petersen 2006.

The pond’s capacity should enable it to supply enough water to meet the demand from crops, integrated farming systems, or other uses for which it is constructed. An economical pond is not oversized relative to either water storage needs or water yield from the catchment.

The seasonal water yield can be estimated using past historical weather data (that is, mean annual, mean seasonal, or certain probability-based rainfall [in millimeters] multiplied by a runoff coefficient, which is usually between 0.1 and 0.3, and multiplied by catchment area). Conservatively, at least 20 percent of seasonal rainfall can be expected to run off black soils, and at least 10 percent can be expected to run off red soils, with mild to medium slopes. Where historical weather data are not available, rainfall analysis must be undertaken before estimating the catchment water yield.