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Increasing water for onsite and offsite uses can be a viable objective for management of certain western rangelands. One approach utilizes water harvesting techniques to increase surface runoff by preventing or slowing infiltration of rain. An attractive alternative, where applicable, is to replace vegetation that uses much water with plants that use less so that more water percolates through the soil to streams and ground water. Most sites are too dry to increase water yield in this way; probably less than 1 percent of the western rangelands can be managed for this purpose. However, where annual precipitation exceeds about 450 mm (18 inches) and deep-rooted shrubs can be replaced by shallow-rooted grasses, there is potential to increase streamflows and to improve forage for livestock. Little or no increase can be expected by eradication of low-density brush and pinyon-juniper woodlands. Potentials for improving water yield are reviewed and summarized by vegetation types.

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.

Results from 11 forest cutting experiments on small watersheds at the Coweeta Hydrologic Laboratory in western North Carolina are presented as a summary of twenty years experience in water yield studies. Conversion of mature forest to low-growing vegetation in this well-watered area increased the supply of water to streams in amounts varying from 5 to 16 inches per year. Increases in annual yield the first year after cutting mature hardwood forest in several ways were roughly proportional to the percentage of the stand cut on any one watershed, but are not yet predictable quantitatively because of variation in soils, physiography, and orientation of small drainages. Increases tend to diminish in the years following the initial treatment, whether cutting is repeated or not, but a certain portion of the increases can be maintained indefinitely by annual recutting. In general, increases are greatest during those months when normal streamflow is lowest, but water yield can be influenced during all se...

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.