World Aquaculture 61 Ponds in drought Forrest Wynne1 Watershed or runoff ponds must be constructed with the proper amount of watershed to fill them under normal weather conditions. Average annual rainfall, soil types, land slope, vegetative cover and climate are some of the factors that will determine how much watershed will be required to provide approximately 1,223,500 L of water. The watershed must be able to provide enough water volume to fill the capacity of the pond basin. Watershed ponds are affected by drought in a number of ways. The loss of water volume and diminished pond depth during extended drought is most obvious. Deep watershed ponds (3.6 m maximum depth, or greater) may be built to contain extra water capacity to compensate for anticipated water loss during hot, dry weather. Relative to surface area, many runoff ponds contain inherently large water volumes and are deep because of the hilly topography. Most warmwater fish production occurs in the 1.2 to 1.8 m of water located near the surface. Under normal conditions, ponds with a maximum depth of more than 2.4 m offer little benefit to fish production. However, extra water volume may be desirable where ponds are exposed to prolonged dry weather. Ponds located in arid lands may be constructed to maximum depths of 3.6 to 4.2 m (Mattinson and Glasscock 1997). During drought, shallow ponds may dry up or fish may die from compromised water quality. Shallow ponds that can be readily topped off with ground or surface water may not need the extra capacity. The capacity of irrigation, livestock, hydrants and some reservoirs may be maximized relative to the pond’s surface area to supply large amounts of water during dry conditions. Seepage and Evaporative Water Loss Many ponds leak small volumes of water either constantly or periodically. Excessive pond seepage may result when ponds are constructed in inadequate or poor locations or they are improperly built. Poor subsoils containing too much sand, gravel, silt, rock formations or too little clay may allow for excessive seepage under normal weather conditions. Water may seep through the basin of ponds where the clay barrier is not adequate to provide water retention. Some pond dams are constructed without adequate topsoil removal, which prevents proper sealing and compaction at the base. Water may leak from under the dam and, in severe cases, may cause a collapse. Some soil types require the construction of a core trench to anchor the dam into the sub soils. Quality clay soil is compacted into the trench and core of the dam to prevent seepage and possible dam failure. Ponds may lose water around plumbing structures, such as drain and overflow pipes installed in the dam. Anti-seep collars should be placed on all drain pipes and other plumbing built into dams. These barriers prevent water movement along the outside of pipes, which may compromise dams integrity. Large trees and shrubs growing on dams may cause seepage by the piping of water along root structures and may eventually weaken the embankment. Woody vegetation growth should be prevented on dams. During some years, evaporative water loss may be compensated largely by direct rainfall into the pond in humid environments (Boyd 1990). However, dry season water loss may not be replenished with direct rainfall until months later. Ultimately, watershed runoff must supply the most timely water replacement during the warm season and the majority of the volume throughout the year. Physical and Biological Effects of Low Water Levels Pond shorelines exposed by receding water levels during drought may create a number of pond management problems along with a few opportunities for pond managers. With large portions of the pond basin exposed, the clay basin liner may develop deep cracks. Marginal clay barriers may become damaged and seep upon re-flooding or seepage problems in ponds that already leak may become more severe. Low pool levels offer some opportunities to renovate dams and remove some silt and debris when the basin can support heavy equipment. Care should be taken to avoid damaging the clay liner during such renovations. Properly repair any damaged areas of the basin with compacted blankets of quality clay soil. Water quality should be carefully monitored during low water conditions to maintain fish populations. Dissolved oxygen depletions may become more frequent and more severe because of the elevated temperature of shallow water and as organic material, such as plants, decompose. Aeration devices may be required to maintain adequate dissolved oxygen concentrations (>5 mg/L) particularly in ponds where fish are heavily stocked and fed (>1,000 kg/ ha). Increased water temperature, pH and reduced water volume may lead to elevated concentrations of toxic unionized ammonia gas (NH3). Feeding may be restricted and supplemental water added to the pond, if available, in an effort to reduce un-ionized ammonia concentrations. Low water levels may reduce or eliminate shallow water nursery habi-
RkJQdWJsaXNoZXIy MjExNDY=