Ground-water outflow from Chino Basin, Upper Santa Ana Valley, southern California

Ground-water outflow from Chino basin was calculated ,by a direct method using the equation Q = PIA, in which Q is the quantity of ground-water outflow, P is the average coefficient of permeability of the sediments through which the flow occurs, I is the average hydraulic gradient, and A is the cross-sectional area of the sediments through which the flow occurs. The period selected for the calculation was 1930-66.

Permeability of the water-bearing sediments was calculated from aquifer test data and from computations involving specific-capacity data from 200 wells in the outflow area. Permeability ranged from less than 100 to more than 5,000 gallons per day per square foot.

The annual hydraulic gradient was derived from contour maps of average water levels in wells for each water year for the period 1930-66.

The cross-sectional area used to calculate ground-water outflow from Chino basin extends southwestward from Pedley Hills to Puente Hills. The area of the outflow section is the saturated thickness of permeable materials measured along the line of section. Part of the lower boundary is the interface between the alluvium and the underlying basement complex, and part is a change in permeability within sedimentary rocks. Geological methods were combined with geophysical methods to determine the cross-sectional area of the water-bearing sediments. Gravity and seismic traverses, drill-hole logs, and data from a more than 600 drill holes, including eight test holes drilled as a part of this investigation, were used to delineate the size and the shape of the outflow area. For the period of calculation, 1930-66, the total area of the outflow section varied from about 16 to 22 million square feet. The fluctuation in total area is caused by changes in the altitude of the water table.

Annual ground-water outflow from Chino basin calculated by the direct method for the period 1930-66 ranged from 38,000 acre-feet in the 1941 water year to 9,400 acre-feet in the 1966 water year.

Two indirect methods of calculating ground-water outflow were studied as a part of this project: the chemical method and the water-budget method. The chemical method was found to be unsatisfactory. Although the ground-water discharge into the Santa Ana River from Chino basin is quite different in chemical composition from the discharge from Temescal basin, there is no known way to quantitatively separate the total discharge with respect to source. In the water-budget method direct runoff and evapotranspiration were reevaluated, and the ground-water outflow from Temescal basin was calculated by the same direct method employed for Chino basin. Annual ground-water outflow from Temescal basin calculated by the direct method for the period 1930-66 ranged from 11,000 acre-feet in the 1940 and 1945 water years to 3,000 acre-feet in the 1965 water year. Annual ground-water outflow from Chino basin computed by the water-budget method for the period 1933-63 ranged from 45,000 acre-feet in the 1941 water year to 10,000 acre-feet in the 1963 water year.