J. F. Hogan

Groundwater recharge in a desert environment: the Southwestern United States.

This American Geophysical Union Water and Science Application monograph is a remarkable compendium of useful information on a key area that is currently lacking in the scientific literature: recharge of groundwater in arid and semiarid environments. As noted in this highly technical work, current and projected population growth and increasing demands on water supply make improved conceptualization and quantification of recharge processes critically important for sustainable management of groundwater resources and the natural environment. More than 40 authors from academia and government contributed.

Environmental tracers applied to quantifying causes of salinity in arid-region rivers: Results from the Rio Grande Basin, Southwestern USA

The upper Rio Grande extends ∼1,200 km between its headwaters in southern Colorado (USA) and the USA/Mexico border region. Over this distance the total dissolved solids content of the water increases from ∼40 mg L−1 to over 2,000 mg L−1. This increase has previously been attributed to evapotranspirative concentration and to flushing by irrigation water of salts accumulated by pre-irrigation evapotranspiration. We employed environmental tracers, including σ18O, σ2H, Cl−, and the Cl/Br ratio to help identify the causes of salinization. Both σ18O and σ2H are enriched with flow distance due to evaporation, but not sufficiently to explain most of the salinization as caused by evapotranspiration. The Cl/Br ratio increases from about 50 in the headwaters to over 1,000 at the distal end of the river basin, indicating influx of subsurface saline waters, which are commonly characterized by high Cl/Br ratios. The Cl− concentration and the Cl/Br ratio increase in a stepwise fashion and are typically localized at the southern ends of the sedimentary basins comprising the Rio Grande Rift, suggesting that the salts are from discharge of deep ground water where it is forced to the surface by bedrock highs, rather than due to flushing by irrigation water.

Geologic origins of salinization in a semi-arid river: The role of sedimentary basin brines

Semi-arid and arid rivers typically exhibit increasing salinity levels downstream, a trend often attributed to irrigated agriculture, primarily due to evapotranspiration. In contrast, the results of our investigations in one salinized river suggest that geological sources of salt added by groundwater discharge are more important than agricultural effects. We performed detailed synoptic sampling of the Upper Rio Grande– Rio Bravo, an arid-climate river with signifi cant irrigated agriculture, and identifi ed a series of salinity increases localized at the distal ends of sedimentary basins. Using Cl/Br, Ca/Sr, 87 Sr/ 86 Sr, and 36 Cl/Cl ratios and δ 234 U values as environmental tracers, we show that these increases result from localized discharge of high-salinity groundwater of a sedimentary brine source. These groundwater fl uxes, while very small (<1 m 3 s –1 ), are the dominant solute input and, combined with downstream evapotranspirative concentration, result in salinization. Furthermore, 36 Cl/Cl ratios and δ 234 U values for these brines are close to secular equilibrium, indicating brine ages on the order of millions of years. The recognition of a substantial geologic salinity source for the Rio Grande implies that alternative salinity management solutions, such as interception of saline groundwater, might be more effective in reducing salinity than changes in agricultural practices.

Geochemical Quantification of Semiarid Mountain Recharge

Analysis of a typical semiarid mountain system recharge (MSR) setting demonstrates that geochemical tracers help resolve the location, rate, and seasonality of recharge as well as ground water flowpaths and residence times. MSR is defined as the recharge at the mountain front that dominates many semiarid basins plus the often-overlooked recharge through the mountain block that may be a significant ground water resource; thus, geochemical measurements that integrate signals from all flowpaths are advantageous. Ground water fluxes determined from carbon-14 ((14)C) age gradients imply MSR rates between 2 x 10(6) and 9 x 10(6) m(3)/year in the Upper San Pedro Basin, Arizona, USA. This estimated range is within an order of magnitude of, but lower than, prior independent estimates. Stable isotopic signatures indicate that MSR has a 65% +/- 25% contribution from winter precipitation and a 35% +/- 25% contribution from summer precipitation. Chloride and stable isotope results confirm that transpiration is the dominant component of evapotranspiration (ET) in the basin with typical loss of more than 90% of precipitation-less runoff to ET. Such geochemical constraints can be used to further refine hydrogeologic models in similar high-elevation relief basins and can provide practical first estimates of MSR rates for basins lacking extensive prior hydrogeologic measurements.

Seasonalizing mountain system recharge in semi-arid basins-climate change impacts.

Climate variability and change impact groundwater resources by altering recharge rates. In semi-arid Basin and Range systems, this impact is likely to be most pronounced in mountain system recharge (MSR), a process which constitutes a significant component of recharge in these basins. Despite its importance, the physical processes that control MSR have not been fully investigated because of limited observations and the complexity of recharge processes in mountainous catchments. As a result, empirical equations, that provide a basin-wide estimate of mean annual recharge using mean annual precipitation, are often used to estimate MSR. Here North American Regional Reanalysis data are used to develop seasonal recharge estimates using ratios of seasonal (winter vs. summer) precipitation to seasonal actual or potential evapotranspiration. These seasonal recharge estimates compared favorably to seasonal MSR estimates using the fraction of winter vs. summer recharge determined from isotopic data in the Upper San Pedro River Basin, Arizona. Development of hydrologically based seasonal ratios enhanced seasonal recharge predictions and notably allows evaluation of MSR response to changes in seasonal precipitation and temperature because of climate variability and change using Global Climate Model (GCM) climate projections. Results show that prospective variability in MSR depends on GCM precipitation predictions and on higher temperature. Lower seasonal MSR rates projected for 2050-2099 are associated with decreases in summer precipitation and increases in winter temperature. Uncertainty in seasonal MSR predictions arises from the potential evapotranspiration estimation method, the GCM downscaling technique and the exclusion of snowmelt processes.

RIPGIS-NET: a GIS tool for riparian groundwater evapotranspiration in MODFLOW.

RIPGIS-NET, an Environmental System Research Institute (ESRIs) ArcGIS 9.2/9.3 custom application, was developed to derive parameters and visualize results of spatially explicit riparian groundwater evapotranspiration (ETg), evapotranspiration from saturated zone, in groundwater flow models for ecohydrology, riparian ecosystem management, and stream restoration. Specifically RIPGIS-NET works with riparian evapotranspiration (RIP-ET), a modeling package that works with the MODFLOW groundwater flow model. RIP-ET improves ETg simulations by using a set of eco-physiologically based ETg curves for plant functional subgroups (PFSGs), and separates ground evaporation and plant transpiration processes from the water table. The RIPGIS-NET program was developed in Visual Basic 2005, .NET framework 2.0, and runs in ArcMap 9.2 and 9.3 applications. RIPGIS-NET, a pre- and post-processor for RIP-ET, incorporates spatial variability of riparian vegetation and land surface elevation into ETg estimation in MODFLOW groundwater models. RIPGIS-NET derives RIP-ET input parameters including PFSG evapotranspiration curve parameters, fractional coverage areas of each PFSG in a MODFLOW cell, and average surface elevation per riparian vegetation polygon using a digital elevation model. RIPGIS-NET also provides visualization tools for modelers to create head maps, depth to water table (DTWT) maps, and plot DTWT for a PFSG in a polygon in the Geographic Information System based on MODFLOW simulation results.

Water quantity and quality challenges from Elephant Butte to Amistad

Management of the Rio Grande between Elephant Butte and Amistad Reservoirs involves a number of distinct water quantity and quality challenges. These challenges arise from both “natural” stressors, such as climate variability and geologic sources of salinization, and “human” stressors, such as bacterial and nutrient contamination from urban storm water, agricultural return flows, and wastewater treatment plants (WWTPs), altered hydrology, and increasing/changing water demands. The discussion does not argue for a single approach to surface water “sustainability” as these decisions are properly made by elected officials and decisions makers. Rather it provides an overview of the surface water system contrasting the pre-development system with the present day to help inform how the stressors mentioned above impact this resource. Current efforts to address these issues are also discussed.