The process of designing a small dam or water pans starts with a water demand analysis to establish the water needs that the structure is supposed to meet. This article will review the process of conducting such a water demand analysis
1. Introduction
An analysis of the water demand of the project area is the starting point of design section of a dam report. The analysis is used to justify the size of the dam or water pan. Here, to the extent possible, the size of the dam or water pan should be able to meet the projected water demand of the project. The figures used in the water demand analysis are usually derived from census data, which offers population data and other relevant information that can be used to estimate the water demand. Reference is also made to the National Water Master Plan, which provides data on the estimated water uses per person, household, livestock and even irrigation. For small dams, the process of conducting a water demand analysis follows the criteria laid out in the Practice Manual for Small Earth Dams and Water Pans in Kenya. The manual recommends estimating the domestic, livestock, and agricultural water demands when constructing dams and water pans.
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2. Domestic/household water demand
Domestic water demand is projected from the population data of the project area. An assumption is made regarding the total area covered by the users of the dam both upstream and downstream. Next, from the population density of the project are is derived from the census data. The population density is then multiplied with the assumed project area to determine the number of users of the dam. The population data is then projected over a period of 1, 10 and 20 years, based on the equations provided in the Practice Manual for small dams. The projections for the first year are referred to as the domestic demand for the design year, while that of the 10th year are termed as future domestic water demand. The figures for the 20th year are termed as the ultimate water demand for the design life of the project and are assumed to be the total domestic water demand for the dam.
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3. Livestock Water Demand
Livestock water demand can be obtained by either; performing a manual census of the livestock of the project area or by making projections from the population of the area. The latter is used more commonly compared to the former. When projecting the livestock data from the population, an assumption is made on the number of livestock that a household in the project area holds, based on local information. The information is then multiplied with the household population density derived from the census data, to establish the estimated number of livestock in the project area. Important to note here is that the livestock in most area comprises of mostly indigenous breeds. Thus, the number of indigenous cows obtained has to be converted into Livestock units based on the conversion rates provided in the Practice Manual for Small Dams and Pans of Kenya. The number of livestock units is then multiplied by the water consumption rate (usually 50 liters per livestock unit) to obtain the total water consumed by livestock in the project area-this is the livestock water demand. Another important point to not here is that livestock data is not projected over 1, 10 or 20 years as is the case with domestic demand. It is assumed that the number of livestock will remain the same over the design life of the dam.
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4. Agricultural/Irrigation Water Demand
Agricultural water demand is estimated from the area that the projects targets to supply water for farming. The area is then multiplied by appropriate water consumption rates based on the method of farming or irrigation adopted in the target area. These consumption rates are provided in the Practice Manual for Small Dams and Water Pans of Kenya. Here, the manual provides for a consumption rate of 60, 90 and 120 cubic meters/hectare/day for drip, overhead, and surface irrigation methods. Alternatively, the water demand can be estimated much more accurately by using FAO CropWat and ClimWat software, which are used to estimate the amount of water consumed in an irrigation project.
5. Evaporation and seepage losses
Given most small dams are exposed to the environment, evaporation losses have to be determined to establish the exact amount of water that is likely to remain in the dam. Evaporation losses are estimated based on data and equations from the Practice Manual for Small Dams. The equations estimate evaporation losses based on the assumed surface area of the dam based on survey data and appropriate rate of evaporation information of the project area. Similarly, given that small earth dams are not lined at the bottom, seepage losses have to be determined to know the exact amount of water that will remain in the dam after they occur. Just like evaporation losses, seepage losses are determined based on equations provided in the Practice Manual for Small Dams and Pans. The equations estimate seepage losses based on the bottom surface area of the dam obtained from survey data and multiplied by an appropriate hydraulic conductivity rate for the project area.
Take a free online course on small earth dam/water pan design here
Buy the Practice Manual, Water Master Plan and other Agric Eng. Reference docs here
Buy a complete earth dam design report here
Buy a complete water pan design report here