Brad D. Wolaver • John M. Sharp, Jr. • Juan Manuel Rodriguez

Abstract: This research develops a procedure for the delineation of regional groundwater flow systems in arid karstic basins with sparse hydrogeologic data using surface topography data, geologic mapping, permeability data, chloride concentration of groundwater and precipitation, and measured spring discharge data. Aquifers are characterized using geographic information systems (GIS) for groundwater catchment delineation, an analytical model for interbasin flow evaluation, a chloride‑balance approach for recharge estimation, and a water budget for mapping contributing catchments over a 45,000km²‑region. The study area includes the Cuatro Cienegas Basin of Coahuila, Mexico, a National Biosphere Reserve containing dozens of springs supporting groundwater dependent ecosystems and irrigated agriculture. Sustainable groundwater development is a key issue on the U.S.‑Mexico border; however, these procedures may be used in arid karstic basins globally.

Keywords: Groundwater recharge/water budget, groundwater flow, water supply, arid regions, karst, carbonate rocks, Cuatro Cienegas, Mexico

Introduction

The objective of this study is to map regional groundwater flow systems in the arid Cuatro Cienegas Basin karstic aquifer. Wolaver et al. (2005) calculate recharge volume on a 870km² intra‑basin recharge area which Lesser y Asociados (2001) suggest produces all spring discharge; recharge calculations generate 0.215Mm³ discharge compared to observed 53Mm³, suggesting interbasin flow. Johannesson et al. (2004) identify high elevation recharge zones from stable isotopes, but not from which catchments. This research tests the hypothesis that most Cuatro Cienegas Basin spring discharge originates as mountain recharge from outside of the basin. Due to the paucity of regional hydrogeologic data, other well-studied carbonate terrains are considered, including the Great Basin Province of the western U.S., the Trans‑Pecos of west Texas, and the Edwards Aquifer of Central Texas.

Methods

A regional flow system is delineated using surface topography, geologic maps, an analytical model for interbasin flow determination, and chloride‑balance recharge estimation using a water budget approach:

R = P C_p / C_r                                                                           (1)

where R=groundwater recharge, P=precipitation, C_p=bulk precipitation chloride concentration, and C_r=groundwater chloride concentration. Groundwater catchments are mapped in GIS using the eight direction pour point model with a 90m² digital elevation model (DEM). Carbonate rocks forming uplands are delineated from geologic maps. Groundwater flow between basins under topographic divides is evaluated using the analytical model:

(2)

where h(x)=hydraulic head at any point(x) in an unconfined aquifer with heads(h₁andh₂) on either side of a topographic divide of hydraulic conductivity(K), length(L), and constant recharge(W_s). A steady‑state water budget is assumed for predevelopment conditions, where inputs are chloride-balance estimated recharge (accounting for evapotranspiration); outputs are spring discharge measured by this project (assuming negligible regional open water evaporation). Regional flow is mapped using a water budget approach; groundwater recharge for successively higher elevation groundwater catchments is summed until observed basin discharge is equal to estimated recharge.

Results and Discussion

Results suggest: 1) groundwater discharges at Cuatro Cienegas Basin because it is the lowest regional elevation; 2) interbasin groundwater flow occurs because topographic divides comprised of highly‑permeable carbonates do not represent groundwater divides (Figure 1) and; 3) up to six hydrogeologically linked groundwater catchments in a 160‑km long flow system discharge in the Cuatro Cienegas Basin. Approaches developed by this research can be used to characterize arid karstic basins with sparse hydrogeologic data globally.