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. It is important to consider three factors in the siting of farm ponds: soil type, topography, and catchment sizes.
For construction of a roadside farm pond, the soils preferably should have low hydraulic conductivity with minimum seepage and percolation so that water can be retained for a longer time in the pond. Soils with a low infiltration rate are most suitable for pond construction. Fout! Verwijzingsbron niet gevonden.10.1 shows the infiltration rate of different soils. Clay and clay-loam soils have good potential for rainwater harvesting without lining, and the seepage losses are minimum. In such areas capturing runoff in ponds and existing depressions occurs naturally. Soils for pond construction should preferably meet the following criteria:
- Soils at the pond site must be fairly impervious in the pond bottom.
- Soils to construct a pond must be compactable. Gravelly soils, sandy soils or soil with certain clays are not suitable.
- The best soils are sandy clay, sandy clay loam, or clay loam.
When geotextile liners are considered, it is useful to standardize the dimensions of the ponds so that the geotextile lining can be pre-prepared rather than being made on the site, which is more laborious and cumbersome.
Table 10.1. Infiltration rates of different soils
|No.||Soil type||Infiltration rate in mm/hr|
|3||Fine sandy loam||12|
Soils having outcrops and stones must be avoided for the digging of farm ponds. The soil profile depth must be investigated before digging the pond. Soils having a good depth of >1 m, free of stones, low pH, low electrical conductivity (EC) and groundwater level may be selected to site the farm pond. Other soils may be problematic. Peat soils have special problems since they are usually very acidic in nature and need copious liming. Soils rich in limestone create problems of precipitating phosphate and iron.
Soil depth is also an important factor during the development of farm ponds. Deep soils have the capacity to store harvested water for a longer duration. Soils measuring more than 1 m are ideal for the construction of farm ponds. The greater the soil depth, the greater the depth of farm pond: a deeper pond will reduce evaporation losses. For farm ponds, a depth of 2.5 m is often recommended: it ensures adequate volume of storage, low evaporation, and ease of access.
Topography is important for a number of reasons:
- It can affect the size, shape, and depth of a pond.
- It affects dam/pond embankment height.
- It affects the speed and intensity of runoff into the pond.
- It greatly affects the simplicity or complexity of pond design and construction and thus cost.
- It directly affects safety issues.
The topographic features of the farm catchment vary from place to place. In general, the proposed land for pond construction must have minimum earth excavation so that costs can be reduced with increased storage. 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 but a U-shape bund can create the pond. In some cases the road body itself can be part of the pond.
The drainage/catchment area that produces runoff for storage of farm ponds is very important. Road construction has a major effect on a catchment’s drainage. Basically, the road infrastructure reshapes the catchment. Roads in general tend to combine smaller sub-catchments and bundle runoff in a limited number of drainage canals. This effectively enlarges the source area of the pond.
In pond-site selection, excessively large source areas should be avoided, particularly if the rainfall is concentrated in a short period (80 percent in less than two months). They increase the sizes required for the levees and translate into higher construction costs. More important, oversized source areas may result in washouts and flushing. Ponds with an oversized source area require spillways and other water-control structures, and are difficult to manage. In such cases a cascade of ponds maybe useful, where water overflows from one storage into the next. If rainfall and runoff are more evenly distributed over the year, pond sizes can be smaller.
On the other hand, the pond must be filled at least once in the season so that farmers can use the water for critical irrigation and other uses 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. Ponds with too small a catchment will have difficulty in filling up; and if the drainage area is too small in relation to the pond size, the pond may not fill adequately, or the water level may drop too low during extended periods of hot, dry weather.
Box 10.2. Siting of roadside farm ponds
- Selection of the site for farm pond depends on local soil conditions, the topography of area, drainage capacity, infiltration, rainfall pattern and distribution.
- Identification of natural depressions where rainwater/runoff either flows or accumulates during the rainy se A good pond site contains: (a) level topography that provides for economical construction, (b) soil with sufficient clay to hold water, and (c) 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 soil, which minimizes seepage.
- Coarse-textured sandy soils should be avoided: these are highly permeable and water will drain through If seepage is believed to be high, they can be plastered with clayey soil and compacted with tree trunks 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: they are usually highly acidic in nature and need sufficient liming.
- In terms of topography, locate a farm pond where there is enough road catchment that runoff can be generated, collected and directed with gravity. Ensure that the ponds can be supplied with water: the road embankment guides it either from road drains or culverts or from the runoff in drainage cuts. In unpaved roads, water exiting at bends and low points, guided by water bars and rolling dips, may be used
- For embankments and their compaction, soils having a wide range of grain sizes are preferable to soils with relatively uniform particle size.
- Before making the final site selection, one should examine potential sites by considering economics, accessibility and safety. Economically speaking, a pond that provides the largest volume of water with the least amount of landfill should be constructed. Liability is also a final consideration. For example, what would happen if the pond or dam fails, causing loss of life or injury?
- Ensure that no water is harvested in farm ponds that are polluted or contaminated.
- Provisions must be made for a pipe and emergency spillway if necessary. Runoff flow patterns must be considered when locating the pond/pit and placing the spoil.
Based among others on Nissen-Pedersen (2006)