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Article Excerpt
ABSTRACT: Precipitation that falls in a watershed undergoes three hydrological processes: evapotranspiration, groundwater recharge, and fiver runoff Using the long-term average stream discharge of a watershed, one can estimate the long-term average evapotranspiration portion of the precipitation by using the precipitation minus the normalized river discharge. In addition, by partitioning the stream discharge into the runoff and groundwater baseflow components, one can estimate the portion of the precipitation that recharges the groundwater and the portion that becomes direct runoff The application variations of the two classical groundwater recharge calculation methods based on the stream discharge, i.e., seasonal recession and recession curve displacement, are discussed for this purpose. A water budget method developed by the New Jersey Geological Survey is also used in this paper for comparison. The calculation of the fate of precipitationn from the average of nine watersheds shows that on average approximately 50% of the annual precipitation is evapotranspired, 25% of the annual precipitation recharges the groundwater, and about 25% of the annual precipitation becomes runoff to area streams.
KEY WORDS: Precipitation, groundwater recharge, runoff, evapotranspiration, seasonal recession method, and recession curve displacement method.
INTRODUCTION
From the discussion of the hydrological cycle in hydrological or environmental geology textbooks, part of the precipitation that falls in a watershed becomes runoff to streams, part of it recharges the groundwater through infiltration, and part of it reenters the atmosphere by evapotranspiration (Fetter, 2001; Pipkin and Trent, 2003). However, rarely does a textbook show exactly what proportion of the precipitation goes to the evapotranspiration, runoff, or, groundwater recharge. It is known that in a hot and arid region, a high portion of precipitation goes to evapotranspiration; in a mountainous area, a high portion of precipitation goes to runoff; and in a humid region, a high portion of precipitation goes to groundwater recharge (Lerner et al., 1990). However, partitioning the exact portion of precipitation is never a clear-cut, simple technique. In estimating the fate of the precipitation from the stream discharge, the calculation of evapotranspiration is the only straightforward component. The evapotranspiration can be determined by subtracting the normalized river discharge in a watershed from the precipitation, if there is no long-term storage change in a watershed. There are various methods that one can use to estimate the groundwater recharge (Urea and Egboka, 1987; Lerner et al., 1990). Two popular, inexpensive and independent methods, which use the stream flow partition techniques, are the seasonal recession method by Meyboom (1961) and the recession curve displacement method by Rorabaugh (1964). These two methods basically use the concept that stream discharge can be partitioned into the contribution from direct runoff and contribution from groundwater baseflow (Figure 1) in a watershed. The Meyboom method identifies one large recharge and one large recession trend in a 12-month cycle utilizing multiyear data, while the Rorabaugh method identifies multiple significant recharge events in a 12-month cycle. A water budget method, which is developed by Charles et al. (1993) of the New Jersey Geological Survey, is also discussed for comparison. This latter method does not use the stream discharge in the watershed; rather, it uses weather, soil and vegetation coverage characters to calculate the runoff and evapotranspiration portion of the precipitation.
[FIGURE 1 OMITTED]
Although the principles of the Meyboom and Rorabaugh methods are clear, the selection of daily and monthly river discharges for estimation of the ground recharge in the application still needs further examination. This paper systemizes the procedures for estimating the fate of precipitation and discusses the proper alternative applications of the Meyboom and Rorabaugh methods for calculating the groundwater recharge rate. The applications provide numerical values on the fate of precipitation in New Jersey. Knowing the groundwater recharge rate is critical for setting any quotas of a wastewater reuse irrigation project or sitting of a septic tank system (Charles et al., 1993).
The 87 precipitation data series used in this paper are from the on-line data base of the National Climate Data Center (NCDC) of National Oceanic and Atmospheric Administration (NOAA) (www. NCDC.NOAA.gov). The nine river gauge data series used in the paper, in turn, are obtained from the United States Geological Survey (USGS) (Figure 2).
[FIGURE 2 OMITTED]
ANNUAL EFFECTIVE UNIFORM DEPTH OF PRECIPITATION
Although the variation of precipitation in the nine watersheds we selected is not very significant, it is necessary to obtain...
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