The main factors in designing a roadside farm pond are the capacity of the water 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
The 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. The seasonal water yield can be estimated using past historical weather data (i.e., mean annual, mean seasonal, or certain probability-based rainfall [mm] multiplied by runoff coefficient, which is usually 0.1 to 0.3, and multiplied by catchment area).
The pond’s capacity depends on the catchment size and factors affecting its water yield. The pond should have sufficient capacity to meet the demand of the crops or integrated farming system for which it is constructed. Using a conservative estimate, a dependable minimum value of 20 percent of the seasonal rainfall can be expected to go as runoff in the case of black soils and 10 percent in the case of red soils with mild to medium slopes.
Storage losses such as seepage and percolation will also influence the pond’s storage capacity. The type of soil in the catchment area contributes to siltation; this must be considered because it affects the pond’s storage capacity. Natural soil-type seepage losses (in mm/day) are presented below, according to FAO 1981.
Table 10.2. Natural soil-type seepage losses
|Soil type||Seepage Loss|
In addition, a suitable provision should be made for the loss in storage capacity due to silting, which is generally kept as 5-10 percent.
Estimating Water Demand
Water-demand estimation is central to the design of farm ponds because it informs the preferred capacity of the pond. The following formula helps to assess the water demand for applied for irrigation, livestock and human needs.
Ir (m3) = 10 x ETcrop (mm) x Ca (ha)
- Ir: Irrigation water requirements in m3 for the entire dry period
- ETcrop: Crop water requirement in mm during the dry period
- Ca: Area irrigated with water from the reservoir in ha
- Ef: Overall water application efficiency
WL = NL x Ac x T
- WL: Water needed for livestock during the entire dry period in m3
- NL: Number of animals to be watered from the reservoir
- Ac: Average rate of animal water consumption in liters per day per animal
- T: Duration of the dry period in days
Wd = Po x Dc x T
- Wd: Domestic water supply during the dry period in m3
- Po: Users of the reservoir
- Dc: Average rate of water consumption in liters per day per person
- T: Duration of the dry period in days
The demand must be linked to an understanding of the rainfall and runoff patterns. Rainfall is one of the most important and critical hydrological input parameters for the design of farm ponds. Its distribution varies both spatially and temporally in semiarid and humid regions of a country. The quantity of surface runoff depends mainly on rainfall characteristics, such as intensity, frequency and duration of its occurrence.
- Frequent rains mean that the pond may fill up several times during the year. This will make it possible to have a smaller (and less costly) pond. The timing of the rainfall and filling of the pond in relation to agricultural requirements are important.
- High, intense rainfall exceeding the infiltration capacity of soil can produce more runoff than the rainfall event with low intensity for longer duration
Rainfall analysis is very critical for optimal economic design of farm ponds (see Annex 3). However, because long-term data on rainfall intensity are seldom available in the country, this available rainfall must be estimated based on probability analysis.
Design rainfall (DR) is defined as the total amount of rain during the cropping season at or above which the catchment area will provide sufficient runoff to meet the crop’s water requirements. If the actual rainfall in the cropping season is below the design rainfall, there will be moisture stress for crops. If the actual rainfall exceeds the DR, there will be surplus runoff that may cause damage to the structures. Timing is important: the creation of storage, as in the development of ponds, will transfer water from peak periods to periods of scarcity (see Annex 3).
DR is calculated from the probability analysis. It is assigned some probability level of occurrence or exceedance. Suppose the probability of 67 percent is given to rainfall: this indicates that the seasonal rainfall may occur or exceed two years out of three; therefore, the crop’s water requirements would also be met two years out of three in a crop season. The higher the probability of rainfall, the more reliable it is for getting assured runoff into the farm ponds.