CORRELATION AND RESPONSE BETWEEN STREAMFLOW AND PRECIPITATION IN THE SAN DIMAS EXPERIMENTAL FOREST, SAN GABRIEL MOUNTAINS, CALIFORNIA

The objective of this research is to analyze the response of a stream gauge to storms and precipitation in Fern Canyon watershed, which lies in the San Gabriel Mountains just north of San Dimas and La Verne. Streamflow data from 1939 to 1959 was digitized and time-series hydrographs created. Precipitation data from nearby locations was acquired from the USFS, and storm events compared to the streamflow hydrographs. These data are organized by water year; e.g., the 1939 water year runs from Oct 1, 1938 through Sept 30, 1939.

In a typical hydrograph, streamflow records small peaks from about October to January, shows maximum discharge during the wet months of February to May, and declines in dry months of June to September. During most years, the rain gauges detected normal precipitation cycles and on average, the stream gauge reflected individual storms. In terms of stream response to storm events, there were consistent delays of 1 to 2 days between maximum discharge and onset of rain during the dry periods, with shorter delays of ~5 to 24 hours during wet months.

The drier periods following abnormally wet years of 1941, 1943, 1952, and 1958 recorded rapid response to rainstorms. The Fern Canyon gauge showed little to no delay due to the soil saturation increase from previously wet years. Occasionally, the smallest storms were not detected by the stream gauge. Factors that could contribute to variations in soil saturation and precipitation include ENSO cycles and droughts. For example, El Niño cycles include 1951 to 1954 and 1957 to 1959, while La Niña cycles include 1954 to 1956.

Response times between storm events and stream discharge are complimentary but can vary due to several factors: length of drainage channels, gauge location, storm duration, amount of precipitation, and soil saturation. With respect to soil, a delayed response could be due to previously unsaturated soils from drought or dry season that results in greater absorption capacity. The precipitation will infiltrate down the soil column and can reduce the amount of water that reached the gauge at the bottom. A long stream channel upstream from the gauge may produce a more delayed response as well. During years of frequent or extended storms, the stream gauge will most likely experience a very short response time, due to increased soil saturation and higher precipitation rates.