Leveraging Precipitation Modification around Large Reservoirs in Orographic Environments for Water Resources Management

Wondmagegn Yigzaw and Faisa

Abstract

This research explored the possible modification of precipitation around large reservoirs in the Cascade Range and Sierra Nevada Mountains in the Western US where orographic precipitation is dominant. After investigating the hypothesis that an additional and man-made source of moisture, such as an artificial reservoir and irrigated landscapes, can modify pre-dam state of orographic process on the windward side or the convective process on leeward side of the mountain, the result is interpreted in terms of better water resources management for future reservoirs. Mann-Kendall’s trend analysis and Sen’s slope estimator were applied for testing the hypothesis using historical hydrometeorological observations (precipitation, relative humidity- RH, and dew point temperature-DPTP). Four blocks of reservoirs (two from Cascade Range and two from Sierra Nevada) and six individual reservoirs were considered in this analysis. Comparison of post-dam period results for the selected reservoirs showed that atmospheric moisture content on the leeward side has a higher slope of increase than windward side. On the other hand, extreme precipitation (90th percentile and above) was found to have an increasing trend for both windward and leeward side during the post-dam era. A key conclusion of this site specific research article is that there is an indication reservoirs in drier location (leeward side) seem to increase precipitation more than those on the windward side. This means we can make use of this extra quantity of flow volume in post-dam period for a sustainable water resource management. Such approach can be part of climate change mitigation and resilient approach to extreme events. As large reservoirs are big parts of socioeconomic development specially in economically emerging countries around the globe, new approach into dam design and operation is important. This means identifying any impact of reservoirs and land use land cover change then leverage this impact as an advantage rather than otherwise.