1a. Objectives (from AD-416):
Determine the rates of potential groundwater recharge in the sugar cane production system including seepage from irrigation canals and reservoirs. Seepage in these areas is critical for recharging aquifers that supply water to the Island of Maui. Quantifying the rates of recharge is needed to assess water supplies for potential biofuels production.
1b. Approach (from AD-416):
Categorize the existing 75 miles of irrigation canals by similar soils, geology, and slope and whether the canal is lined or unlined. Within specific sample areas of similar geology, slope, soils, and canal type (line or unlined), use geophysical methods to assess potential for seepage. In areas of potentially high seepage, install seepage meters to give more quantitative rates of water loss within the canal system.
3. Progress Report:
The Navy's dependence on oil (petroleum) strains operational planning. Its focus is on securing a sustainable fuel supply. ARS research and models are helping determine best management of natural resources to allow Office of Naval Research (ONR) sustainability in fuel supply while also promoting ecological services and the local economy in Hawaii. Efficient water management in sugarcane production requires an understanding of water losses. In cooperation with scientists at Baylor University and Texas AgriLife Research, seepage under irrigation canals and water supply reservoirs has been identified as potential sources of water loss. A geophysical technique called resistivity was applied to six irrigation reservoirs at varying elevations and geology to determine areas of active seepage. Initial results showed a perched water table approximately 3 feet to 20 feet from the surface. To quantify the rate of seepage, a seepage meter was designed, developed, and applied to the six reservoirs that were analyzed using resistivity. Contrary to current understanding, all reservoirs showed a net influx of water, shifting focus on management of the perched water table to minimize seepage losses. This past year three more reservoirs were surveyed. A ponding study was conducted on a large reservoir underlain with sandy soils. The results showed seepage occurred but at one-third of the previously estimated rate. Also, state-of-the-art Doppler flow meter technology was tested on six canal lengths to determine seepage losses under the canals. With accuracy within 5-10%, the methodology is well suited to stream flow measurements; however, the accuracy may not be adequate for estimating losses in canals. The method is currently being refined to determine if it may provide realistic seepage losses in the canals. Quantifying reservoir and canal losses will allow more efficient management of irrigation water and more sustainable biofuel production in Hawaii.