EFFECTS ON BASEFLOW RECESSION AND EVAPOTRANSPIRATION RATES USING A GIS INVESTIGATION OF PAIRED WATERSHEDS IN THE EASTERN SAN GABRIEL MOUNTAINS

Two paired watersheds, Upper San Antonio and Icehouse Canyons, located in the eastern San Gabriel Mountains were studied using a geographical information system (GIS) approach to examine the effects of rock and soil coverage, vegetation, hill shade, slope, aspect, and solar insolation variables on the baseflow recession and evapotranspiration rates.

Baseflow recession constants for Icehouse and San Antonio Canyons were previously determined to be 0.0080 days^-1 and 0.0185 days^-1 respectively. Other researchers have speculated as to the causes of this difference and it is thought that the differences are partially due to the effects of vegetation, hill shade, differences in rock /soil types and amounts, and solar insolation on the baseflow recessions; however, no quantitative data for these potential variables exists for the study area. An analysis of three different geological maps of the area and four map variables was conducted to determine their effects on baseflow recessions and evapotranspiration. Significant, measurable differences were noted in the types and amounts of vegetation, the configuration of rock and soils present, the aspect for each canyon, and the amounts of solar insolation received by each canyon throughout the year. Aspect, hill shade, and solar insolation values all have an effect on the overall availability of water for plant use and are reflected in the types and locations of various plants seen in the study area. Colluvium, landslides, slope, and covered bedrock also play an important role in the availability of water either for storage and use by vegetation or in groundwater flow.

Evapotranspiration rates for Upper San Antonio and Icehouse Canyons were found to be 55% and 45% of the annual precipitation influx respectively. Several mapped factors play a role in determining the evapotranspiration rates in these canyons. Two major factors are solar insolation and canyon aspect in the two watersheds which supports these rates. South facing Upper San Antonio Canyon receives more solar insolation and reflects the higher evapotranspiration rate in contrast to the bimodal, east-west trending Icehouse Canyon that has a significantly lower evapotranspiration rate.