Ground-water recharge on the southern High Plains, east-central New Mexico
— William J. Stone and Brian E. McGurk

The Southern High Plains extend into eastern New Mexico in three separate areas (Fig. 1). One of these lies in east-central New Mexico. It covers most of Curry County and the southwestern part of Quay County.

Although the High Plains cover only a small portion of New Mexico, they consitute an important agricultural area. According to estimates of acreage cultivated during the period 1973-1978 (Lansford et al., 1979), approximately 200,000 acres of irrigated cropland and 368,000 acres of dry cropland were in production in Curry County, whereas 53,000 acres of irrigated cropland and approximately 238,000 acres of dry cropland were under cultivation in Quay County.

The irrigated portion of this farmland owes its existence to ground water pumped from the Ogallala Formation. This aquifer not only provides water for irrigation, but also for municipalities and rural homes. Since extensive pumping for irrigation began in the 1930's, water levels have dropped substantially. Local declines of 7-9 m during 3-5-year periods have been common (Reeder et al., 1959; Hudson, 1978). Such declines occur because withdrawal rates exceed recharge rates. Safe yield (rate of withdrawal at which undesired results are not produced) may have been reached in Curry and Roosevelt Counties as early as the 1930's (Theis, 1932). Water levels continue to decline, and irrigation has been abandoned in some areas because of excessive lifts and fuel costs.

The rate of ground-water recharge is essential for assessing the life of the ground-water supply. However, recharge data are often not available, and the various chemical and physical methods of determining recharge in common use are complex, time-consuming and expensive. An alternative chemical method, based on chloride content of soil or vadose water, is simple and relatively inexpensive.

In 1983, the chloride method was applied to three settings in Curry County to obtain representative recharge rates and evaluate the variation of recharge with landscape setting on the High Plains of New Mexico (Stone, 1984a). The purposes of this paper are: (1) to modify the local or point recharge rates obtained in that study based on a new measurement of chloride content of local precipitation, (2) to convert these local recharge values to a regional recharge volume for Curry County and (3) to extend these recharge values to similar terrain in southwestern Quay County (the rest of the Southern High Plains in east-central New Mexico).

The region studied includes Curry County and southwestern Quay County (Fig. 1). Throughout this region, thin Quaternary deposits overlie the Ogallala Formation (Miocene and Pliocene). The Ogallala consists of up to 107 m of alluvial, eolian and lacustrine deposits (Hawley, 1984). The major Quaternary unit overlying the Ogallala is the Blackwater Draw Formation (Pleistocene), formerly mapped as windblown cover sand (Texas Bureau of Economic Geology, 1977). This unit consists of eolian sand occurring in a broad blanket generally less than 8 m in thickness. Locally, lacustrine and fluvial materials of Quaternary age also lie at the surface. An excellent summary of the geology and hydrology of the region has been given by Galloway (1982).

The climate of the region is semiarid. Annual precipitation averages approximately 400 mm, and potential evapotranspiration generally exceeds 1000 mm (Gabin and Lesperance, 1977). Most precipitation occurs in the period May through October. Net water-balance deficits of 760 mm or more are common.