Chaparral Shrublands

Streamflow

There are few large watersheds covered solely by chaparral. Since chaparral exists mainly along flanks of mountain ranges, most large drainages containing chaparral extend into conifer forest above or semidesert vegetation below it. Basins lying entirely within the chaparral seldom exceed a few thousand acres.

Annual—Streamflow has been gaged on a number of small experimental watersheds ranging in size from 9 to 3,000 acres. Mean annual water yield on chaparral watersheds varies from less than 0.1 inch per year on the drier sites to 2.5 inches on the wetter areas. For representative chaparral sites Hibbert et al (1974) estimated a mean water yield to be 1.25 inches from a mean precipitation of 22 inches or about 5 %.

Annual streamflow in the chaparral fluctuates more widely than precipitation. In dry years, water yield may be negligible or even absent. In very dry situation a condition develops that Hibbert et al (1974) calls a "negative yield" in the sense that water deficits continue to increase after streamflow stops. This situation can occur due to the relatively large storage capacity of the characteristic deep soil regolith normally found in chaparral areas. Before streamflow (base flow) can resume this deficit must be satisfied. Thus after prolonged drought a greater amount of recharge is necessary to attain a give flow level than in years with rainfall near normal.

While dry years produce little water, wet years, may yield 20 % ( compared to 5% in an average year) of the precipitation or more in some areas (Hibbert et al 1974). One wet year may produce runoff equal to several years of near average precipitation.

Some of the water yielded by small headwater basins is lost to riparian vegetation downstream, the proportion being larger in dry years than in wet years. The amount of the loss will vary according to length and type of stream course. Many intermediated-size watersheds, such Sycamore Creek (119,000 acre) near Fort McDowell, flow intermittently on the surface, although flow may continue in the channel alluvium. Surface flow in the upper part of Sycamore Creek is perennial, but in the lower part all surface flows up to 200 ft3/s soak into the unconsolidated channel fill, which along the lower 9 miles is as much as 100 ft deep. Thomsen and Schumann (1968) estimated that about 4,000 acre-ft per yr discharges to the Verde River through this alluvium.

Winter—The amount and seasonal distribution of streamflow depends largely on when and how much it rains. Most runoff is produced during the cool winter months when ET is low and precipitation is relatively heavy. Average seasonal distribution of water yield is 85% during the November-April dormant season when 55% of the precipitation occurs.

Although summer rainfalls often occur as high intensity storms, very little of the summer rainfall runs off. Summer storms account for 12 % of the rainfall in July, August and September with 35% of the rain (Hibbert et al 1974). The 3 driest months (May, June, and October) account for the remaining 3% of the yield from 10% of the rain. The proportion of rainfall normally intercepted by the chaparral brush is large because rainfall amounts are usually small and rain reaching the ground is readily absorbed in the upper few inches of soil and quickly transpired or evaporated. Headwater streams are often dry by early summer, although recurrent heavy rains may sustain base flow at a low level throughout the summer.