Influence of climate variability on the ecologically sustainable water withdrawals from streams for irrigation
Type of DegreeMaster's Thesis
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Alabama (AL) receives a large amount (1270-1727 mm annual average) of rainfall annually. However, much of the rainfall occurs in the non-crop growing season (winter months), and recurring, severe droughts during the crop growing season lead to losses in crop production. In the past two decades, percentage of cropland area irrigated in AL has increased from 42 to 51%. In AL, in addition to using groundwater for irrigation, farmers mostly withdraw water from streams to irrigate crops. However, if water withdrawal from streams is not done in an ecologically-sustainable manner, it can potentially harm stream ecology and reduce the dilution capacity of streams, and therefore impact water quality and aquatic biota. In the southeast United States (U.S.), the quantity of water that can be ecologically-sustainably withdrawn from streams for irrigation depends on El Niño Southern Oscillation (ENSO), a seasonal-to-interannual (SI) climate-variability phenomena. Therefore, it is important to understand how ENSO affects streamflows and therefore, quantity of water that can be sustainably withdrawn from streams for irrigation. The major goals of this study were to: (a) determine how ENSO forecasts can be used to withdraw water sustainably from streams for irrigation and (b) quantify the effect of upstream water withdrawals from streams on the downstream water withdrawals. The study was conducted in the Swan Creek watershed located in Limestone County, AL, U.S. The Soil and Water Assessment Tool (SWAT) model was used to simulate streamflows and develop water withdrawal prescriptions. The results of this study show that La Niña phase of ENSO generated more rainfall from January to March (non-crop growing season), and the El Niño phase generated more rainfall from May to December (except October) (crop growing season). Irrespective of the ENSO phase, the amount of water that can be sustainably withdrawn from streams during non-crop growing season was two times the amount of water that can be withdrawn sustainably from streams during crop growing season. During non-crop growing season, volume of water that can be sustainably withdrawn from streams was greater during La Niña phase relative to El Niño phase. The results indicate that when water withdrawals based on water withdrawal criteria were made at the outlet of each subwatershed with no water withdrawals upstream, on an average, the percentage of subwatershed area than be irrigated using water withdrawn ranged from 1.4% to 10%. This range depended on season (crop growing vs. non-crop growing) in which water was withdrawn and stream order. The water withdrawals in upstream areas affected downstream flows. For example, at the watershed outlet on an average annual basis, volume of water available for withdrawal reduced by 72% when the water withdrawals were made at the outlet of all the subwatersheds upstream of watershed outlet relative to no withdrawals made upstream of watershed outlet. For a pond with an average depth of 2.13 m (7 feet), surface area required (m2) could be calculated as 0.214 times the area under irrigation (m2). Overall, results of this study show that ENSO forecasts can be used to withdraw water sustainably from streams for irrigation.