Irrigation Dugouts

Joyce Koke, PFRA, Brandon, Manitoba

Dugout storage reservoir for irrigation

Dugouts (also called farm ponds) are water reservoirs excavated into the earth where topography doesnıt allow a reservoir to be created by a dam on watercourse. There are two main factors to consider when determining if a dugout would be suitable for your irrigation water supply.

The first factor is the drainage area. Will the drainage areas supply enough water for your demand and your downstream neighboursı demands?

The second factor is dugout size to do the job. Consideration should be given to evaporation and precipitation when determining size. The dugoutıs ability to store water becomes very important in the spring to hold runoff. The occurrence of runoff is also important. If the majority of runoff occurs in the spring, the dugout must be capable of storing the annual water demand. On the other hand, if the dugout can be refilled from summer flows the dugout size can be reduced.

Pumping to fill irrigation reservoir to a level higher than spring runoff levels

The economics of dugouts for irrigation are very sensitive. The costs per cubic metre (m3) of water storage are very high. Typically with only excavated storage costs are approximately $0.80 per m3. For each 10 cm of water applied, each hectare requires 1,000 m3 of water costing $800 for excavated storage. To store water for a complete yearıs irrigation, about five 10 cm applications per hectare, requires 5,000 m3 of water. Storage construction costs are approximately $4,000 per hectare.

Dugout storage becomes more feasible in larger storage quantities because the excavated material can form a dike to keep the water above original ground level. At greater than 40,000 m3 of storage the excavated earth volume need only be one-half the total stored volume of water. The ratio of stored water to earthwork volume can rise to 3 to 1 at 100,000 m3 of storage. Normally all water stored in a diked portion of the reservoir must be pumped up into storage. This pumping is fast and economical because the high volume low lift pumps need relatively little horsepower.

When considering the economies of both size and favourability of a drainage area i.e. good probability of flows during the irrigation season, dugout storage is feasible. For example a drainage course with a 90% probability of enough flow in each growing season month will provide a 10 cm application of water. In this case, a full seasonıs water storage would not be necessary. Storage for 10 cm applications or an alternative source that can be pumped in dry years would allow a crop in most situations.

An irrigator should be cautious about embarking on a project without sufficient storage. One farmer who irrigated 40.5 ha with only 15,200 m3 of storage encountered a dry year last year and found that he had to pump continuously from an alternate source 3.2 km away. It was a very costly venture and one farmer said he would never do it again. The farmer is increasing the size of his storage. In his situation, storage of 40,500 m3 is recommended. Poorer probabilities of summer refills require greater storage up to the total of 202,500 m3 that was pumped last year onto the 40.5 ha.

Planning and designing an irrigation project takes time and effort. Be prepared to spend roughly a year obtaining permits, licenses and constructing the proposed project. PFRA and provincial departments of agriculture can provide assistance in the planning and design of irrigation projects and should be contacted early in the process. In Saskatchewan, assistance can be obtained from SaskWater.