Search

Water yield has increased substantially on two small chaparral watersheds in central Arizona following brush control and conversion to grass. Winter precipitation is the major source of water yield. Summer rains begin in July and normally account for about one-fourth of the annual precipitation but generally have little effect on streamflow. When annual precipitation is less than 16 inches, increase in water yield resulting from treatment is likely to be less than 2 inches. However, the efficiency of the conversion for producing extra water improves with rainfall, at least up to 34 inches. At this amount of annual precipitation, the increase in flow may reach 12 inches or more, depending on seasonal distribution of the rain.

Digital water level recorders on small, flashy streams are frequently set at 5-minute readout intervals to define fluctuations during critical storm flow periods. Consequently, during nonstorm periods far more data points are punched on the recorder tapes than are needed to define the long-term hydrograph. A two-step reduction process eliminates more than 95% of the original 105,000 data points per year of record. The first reduction is obtained by translating to IBM cards only every twelfth (hourly) head value punched on the tape during nonstorm periods. Storm periods are translated at 5-minute intervals. A computer reduction step on both hourly and 5-minute data then systematically rejects head values not essential to hydrograph definition. Rejection is based on differences in elevation and slope between successive head values. Computer output consists of cards and a listing of the reduced data.

Field measurements of sou moisture content and pressure potential were used to determine the moisture flux, hydraulic gradients, and dynamic conductivity of a 200-foot sloping soil lysimeter. These measurements were then used to predict outflow and evapotranspiration. A vertical unsaturated Darcian analysis was used to evaluate conductivity during the first study phase, in which evapotranspiration was eliminated by sealing the model surface with a plastic sheet. The moisture flux term in the Darcy equation was determined by evaluating moisture content change in depth and time. Moisture flux determined in this way agreed with measurements of actual outflow. Conductivities were solved from flux and hydraulic gradients, and corresponding water contents were assigned. Conductivity curves were highly significant and agreed with those obtained when the model was uncovered. Evapotranspiration was solved during the second study phase (cover removed and grass established) by using a water balance based on moisture content and potential, rainfall, and conductivity (from the first study phase). Results at the deepest level in the profile agreed well with those based on actual outflow.