The analysis of the historical records of streamflow, precipitation and temperature was carried out using singular spectrum analysis. The important result of the space-time decomposition is that the annual spring flood nearly disappears for a 25 year interval (1960-1985). This response is observed in the annual runoff mode which shows a long term reduction in amplitude (1960-1975) with an increase later in the period (1978-1985). No corresponding mode is observed in the precipitation or temperature. Explanation for the reduction in peak flow (spring flood) from 1960-75 seems to be the construction of small upland storage reservoirs which likely changed the relative balance of evaporation and recharge, both increasing at the expense of upland runoff. This increase in recharge and damping of the seasonal runoff response is consistent with the longer time scales of groundwater storage and baseflow to the stream determined in our work. Recovery of the peakflows which began in the late 70’s is also interesting since it starts before the El Nino years of the 1980’s. During this period most of the upland reservoirs have become choked with sediments and the hydrologic system has returned to the pre-1960 condition with “normal” spring floods.
Present activities involve implementation of a soil moisture-groundwater-runoff model of the Little Washita which simulates the observed effects well. Our next step is to validate the model with SGP soil moisture initializations from the ’92 and/or ’94 campaigns. The research shows the importance of the soil moisture-groundwater connection in water resource management, and the difficulty of confirming the relative impacts of climate and landuse change.
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