Scientific Investigations Report 2007–5007

Scientific Investigations Report 2007–5007

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Factors Controlling Recharge

The two dominant factors controlling predevelopment ground-water recharge are the quantity and timing of precipitation (including snow accumulation and melt) and the soil properties. Although recharge is affected by the LULC, it is similarly controlled by the quantity of precipitation. For current conditions, the irrigation of croplands is an additional controlling factor. The daily-to-interannual (climate) variability of precipitation, and to some extent air temperature, also influences the amount of recharge under both predevelopment and current conditions.

The relation between mean annual recharge and total water input (R/T, where T is total water input: precipitation for predevelopment conditions and precipitation plus irrigation for current conditions) clearly shows the control that the quantity of water input has on recharge (fig. 14 and table 2). Estimated recharge increases nearly linearly from T quantities of about 7 to about 40 in. At T values larger than about 40 in., the relationship is nonlinear due to the effects of: (1) timing of water input, (2) excess water above the amount that can be captured by AET, and (3) soil properties. The timing of water input is best reflected by estimated current recharge for two areas (Prosser and Roza-Sunny areas, tables 1–2) with the largest percentage of irrigated lands. These two areas have a total water input of about 53 and 50 in., respectively, and ratios of irrigation to precipitation (I/P) of more than 5.5; I/P ratios ranged from about 0.2 to 4 for the other areas. During the growing season, crops in the Prosser and Roza-Sunny areas use a larger part of T in comparison to the upper Yakima area (which has a similar total water input) because the input to the upper Yakima area mainly occurs during the non-growing season and is not available for AET. This difference also reflects the excess water above the amount that can be captured by AET; note that the potential evapotranspiration is also much lower in the uplands than in the lowlands.

The control that the soils have on recharge is reflected in the relation between mean annual recharge and the ratio of the total quantity of water that can be stored in the root or soil zone (S) to T (fig. 15). The information for figure 15 is only for the areas modeled with DPM, because PRMS does not include a saturated soil-moisture storage compartment. S is used instead of TAWC because S/T values are larger than TAWC/T, and the large amount of irrigation input results in saturated soils (one reason for constructing tile drains, wasteways, and drainage ditches) that are better accounted for by using S. For S/T values greater than one, recharge is limited because the mean annual water input is less than the total water storage capacity, and for values less than one, the estimated recharge sharply increases, especially for values less than about 0.6. The estimated average S for the basin is about 14 in., indicating that, on average, the soils can store about 14 inches of the precipitation falling in the basin. Indeed, the difference between the basin’s mean annual precipitation and S is about 12 in., which is nearly equivalent to the estimated mean annual predevelopment recharge.

Mean annual recharge is greater than zero for S/T values greater than 1.0 because of the temporal variations in the water input. For example, the Prosser area (fig. 8) has a S/T value of 1.65 and a mean annual predevelopment recharge value of 0.11 in. (table 2) with a large range in annual predevelopment recharge—35 percent of the annual values were zero, 68 percent were less than 0.01 in., and 85 percent were less than 0.2 in. (fig. 16). These variations are due to both interannual variations in precipitation and its timing; if the precipitation quantity is large or if it occurs over a short period of time, the TWAC may be exceeded, especially during September through March when potential evapotranspiration is low. The non-linearity between ground-water recharge and daily-to-annual precipitation has been described by Rushton and Ward (1979), Stephens and Knowlton (1986), Gee and Hillel (1988), and Bauer and Vaccaro (1990). The non-linearity was a major factor for using daily modeling techniques.

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