Historical Climate and Streamflow Trends in Santa Ana River Basin

Santa Ana River watershed, located in Southern California, is the home of more than 5 million people. Population is projected to double within the next 50 years in the 2,650 square miles watershed. With prolonged drought conditions, and projected climate change, a strong concern exists about sustainable water supply of the area. In this study, historic climate and streamflow trend from water year 1965 to 2014 is analyzed using the nonparametric Mann-Kendall test. Climate trends are studied using annual rainfall, and annual average maximum and minimum temperature at 5 and 4 weather stations, respectively. Three of the precipitation stations show precipitation is decreasing in the watershed while minimum and maximum temperature has an increasing trend at three stations (p < 0.05). To assess whether streamflow and stream-channel characteristics are tended to increase or decrease monotonically with time, four variables - (1) annual maximum peak, (2) annual mean, (3) low to moderate and (4) moderate to high maximum peak streamflow were tested at 20 stream gauge sites. Only at 5 stream gage stations, significant streamflow trend is observed. At two stream gages, annual peak and annual average streamflow is increasing and at two stations, annual average streamflow has a decreasing trend. Low to moderate peak streamflow is increasing at two gage locations but there is no monotonic trend in moderate to high flows. As precipitation is decreasing in some part of the watershed, the effect of increasing urbanization in the area can be attributed for the localized increase in mean and peak streamflow. The trend analysis in weather and stream gage data will be presented in detail.