Urban water budgets: a comparative study of three landscape management scenarios in southern California

The fate of irrigation water in urban landscapes is highly uncertain, due in large part to uncertainties in urban ecohydrology. In this study, we compared evapotranspiration (ET), leaf-level stomatal conductance, and soil moisture of three landscapes managed under typical, retrofitted, and low-impact scenarios. The typical landscape consisted of fescue (Schedonorus phoenix (Scop.) Holub), a common cool-season turfgrass species. Watering was controlled by a timer and occurred on a daily basis, representative of irrigation by most homeowners. The retrofitted landscape consisted of seashore paspalum (Paspalum vaginatum), a warm-season species, and watering was based on measurements of soil moisture. The low-impact landscape consisted of mixed species of cool-season native sedge (Carex), and watering was based on measurements of weather conditions. The amount of irrigation received was highest in the typical landscape, and lowest in the retrofitted landscape. We hypothesized that the low-impact landscape would have the highest irrigation efficiency or IE (ET/applied irrigation water) while the typical landscape would have the lowest, due to differences in stomatal conductance and soil moisture. ET was measured using a chamber technique, and stomatal conductance was determined using a porometer. Destructive samples were used to estimate leaf area index (LAI) and soil moisture was measured using time domain reflectometry (TDR). We found no significant difference in soil moisture among landscapes, except in spring when the retrofitted landscape was drier than the other two. The average annual ET was similar among landscapes, though there were large seasonal differences. In the fall, the ET of sedge was higher than fescue, and paspalum did not differ from the other two species. In the winter, the ET of the warm-season grass paspalum was lower than the cool-season sedge and similar to fescue. In spring, there were no differences among species. In summer, the ET of paspalum was higher than the other two species, which did not differ. Species differences in stomatal conductance were usually similar to ET, and the highest conductances were from paspalum during the summer. Below canopy vapor pressure deficit was extremely low (<0.5 kPa), suggesting low evaporative losses. Due to high LAI, ET is likely dominated by transpiration. The typical landscape had the lowest IE of 40%. It was unexpected that the low-impact landscape had only a slightly higher IE of 49%, while the highest IE of 68% was of the retrofitted landscape. Even in a high IE landscape, it is surprising that nearly one-third of applied irrigation may be lost as drainage or runoff. Along with the large volume of inefficient water use, drainage and runoff from turfgrass may contain pesticides and fertilizer, adding to possible environmental consequences.