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Studies of the hydrogeology of the southern Great Basin differ widely in their conclusions regarding the origin of major springs at Ash Meadows, in the Amargosa Desert, Nevada, and in the Furnace Creek- Nevares Spring area in Death Valley, California. The diversity of opinion reflects the following. First, ground water commonly moves between intermontane basins of the region via thick, highly fractured, and areally extensive Paleozoic carbonate rocks; the resulting lack of correspondence of topographic and ground-water divides precludes routine utilization of the water-budget method in the study of these basins. Second, subsurface hydraulic data for the regional carbonate aquifer are sparse and difficult to interpret because of the complex subsurface disposition of and hydraulic barriers within the aquifer. An analysis of hydrologic, geologic, geochemical, and isotopic data permits a first approximation of the subsurface watershed tributary to the cited spring groups. Water temperature, chemistry, isotope content, hydraulic head, and geologic relations indicate that the major springs at Ash Meadows and in the Furnace Creek-Nevares Spring area, though emerging from unconsolidated Quaternary strata, are fed by water moving directly from the underlying carbonate aquifer of Paleozoic age. Joint use of potentiometric, geologic, and isohyetal maps indicates that the subsurface watershed tributary to Ash Meadows is no smaller than 4,500 square miles. The Ash Meadows ground-water basin is bordered on the south and east by the Spring Mountains and Sheep Range, the principal recharge areas, and on the west by the Belted Range, Eleana Range, and Shoshone Mountain. A northern boundary was not definable, and some underflow from White River ground-water basin, 90 miles northeast of the springs, is probable. The hydrogeologic data do not support the conclusion of earlier studies that underflow from Pahrump Valley is the major source of the spring discharge at Ash Meadows; probably no more than a few percent of the total comes from that valley. Comparison of the size, climate, and discharge from the Ash Meadows basin with that of the surface watershed tributary to the Furnace Creek-Nevares Spring area indicates that most of the spring discharge in east-central Death Valley originates well beyond its confines. Disposition of the carbonate aquifer favors the movement of ground water into Death Valley from central Amargosa Desert. Water in the carbonate aquifer in the latter area may be derived from the Ash Meadows basin, from the overlying valley fill, or both. Five hydrochemical facies were distinguished by percentage of major cations and anions in ground water from 147 sources. The hydrochemical facies reflect both the mineralogy of strata within recharge areas and downward crossflow from a Tertiary tuff aquitard into the carbonate aquifer. The areal distribution of these facies provides evidence for a northeasterly source of the Ash Meadows discharge, absence of significant underflow from Pahrump Valley to Ash Meadows, and movement of water from the central Amargosa Desert to the Furnace Creek- Nevares Spring area. The data are also compatible with southwestward underflow into the Ash Meadows basin from the White River basin. The deuterium content of 53 water samples from 27 major valleylevel springs and selected wells falls into several areally distinct patterns which suggest that 35 percent of the Ash Meadows discharge is derived from the White River basin, that underflow from Pahrump Valley is unlikely, and that water discharging in the Furnace Creek-Nevares Spring area may be related to water in the carbonate aquifer within the Ash Meadows basin. However, other interpretations are possible indicating that unequivocal interpretations about the regional flow system cannot be made from isotopic data alone.

Work presented in this dissertation was performed in the salt-desert environment of the Owens Valley of California. The area experiences low-rainfall, hot summers, but has a high water table, seldom more than 5 meters from the surface. To test differences in plant species wateruse, a steady-state porometer was used for transpiration measurements while a 2-meter point-frame was used to estimate leaf area index on each species studied. The five species studied (Atriplex torreyi, Chrysothamnus nauseosus, Distichlis stricta, Sporobolus airoides, and Sarcobatus vermiculatus) varied with regard to photosynthetic pathways and leaf morphology. Water-use differences among species are hypothesized to be related to the differing physiological and morphological characteristics observed in the different species studied. This work focuses upon methods for integrating porometric transpiration rates and point-frame measured leaf area to estimate daily plant water-use. Daily water-use values are correlated with environmental growth conditions. A computer program was developed for scenario testing so that conclusions could be drawn concerning how given plants respond to different conditions of soil moisture and atmospheric evaporative demand. The computer-aided calculations led to conclusions that low water-use behavior characterizes A. torreyi, and high water-use behavior characterizes C. nauseosus. C4 photosynthesis and low leaf conductance may contribute to the success of A. torreyi on fine-textured soils when water transfer rates to roots are limiting to transpiration. Fine-textured soils may inhibit production in C. nauseosus because the species requires higher rates of transpiration to achieve optimal growth than soil hydraulic conductivity allows. These conclusions have implications for land managers who should recognize that climax plant communities in saltdesert regions are better at conserving water and stabilizing soil than is colonizing vegetation. Management should seek to maintain climax vegetation cover because restoration is difficult once vegetation disturbance occurs.

Water allocation in the upper Klamath Basin of Oregon and California has been challenging. Irrigators have increasingly turned to groundwater to make up for surface water shortages because of shifts in allocation toward in-stream flows for Endangered Species Act listed fishes. The largest increase in groundwater pumping has been in and around the Bureau of Reclamation's Klamath Irrigation Project, which includes the Tule Lake subbasin in the southern part of the upper Klamath Basin. Previous groundwater flow model simulations indicate that water level declines from pumping may result in decreased flow to agricultural drains in the Tule Lake subbasin. Agricultural drains on the Klamath Project are an important source of water for downstream irrigators and for the Tule Lake and Lower Klamath Lake National Wildlife Refuges. To better assess the impact of increased pumping on drain flow and on the water balance of the groundwater system, flow data from agricultural drains were evaluated to investigate the changes that have taken place in groundwater discharge to drains since pumping volumes increased. Additionally, a fine-grid groundwater model of the Tule Lake subbasin was developed based on the existing regional flow model. The fine-grid model has sufficient vertical and horizontal resolution to simulate vertical head gradients, takes advantage of time-series data from 38 observation wells for model calibration, and allows agricultural drains to be more explicitly represented. Results of the drain flow analysis show that the groundwater discharge to agricultural drains has decreased by approximately 4000 hectare-meters from the 1997-2000 average discharge. Most of this decrease takes place in the northern and southeastern portions of the subbasin. Results of the groundwater model show that the initial source of water to wells is groundwater storage. By 2006, approximately 56% of the water from wells is sourced from agricultural drains.