Estimates of characteristic times to approach steady state flow in multidimensional infiltration in the landscape depend on the magnitude and character of the capillary length scale λc and the associated capillary time scale tc. Here we derive relationships between λc and tc and readily measured field properties sorptivity S and hydraulic conductivity K or S at two supply heads. We explore the relations between λc and tc and other macroscopic and microscopic length, potential, and time scales. In addition, we show that the microscopic characteristic length λm associated with λc gives physically plausible estimates of flow‐weighted mean pore dimensions. We contrast values of λc, tc, and λm for undisturbed field soils with those of repacked materials for water supply potentials close to zero. Large λm for the undisturbed surface soils are attributed to preferential flow. Data from here and elsewhere reveal no apparent trend of λc with soil texture, with most λc of the order of 100 mm. We suggest that the characteristic size of devices used to determine hydraulic properties of field soils should be greater than or equal to λc for representative measurements. The geometric mean time of approach to steady state flow when water is supplied at potentials near or greater than zero is found to be 1.7 hours. This value together with published results suggest that the time of approach to steady state flow from multidimensional cavities is of the order of 1 hour for many field situations.

The disposal of hazardous and radioactive waste in arid regions requires a thorough understanding of the occurrence of soil‐water flux and recharge. Soil‐water chemistry and isotopic data are presented from three deep vadose zone boreholes (>230 m) at the Nevada Test Site, located in the Great Basin geographic province of the southwestern United States, to quantify soil‐water flux and its relation to climate. The low water contents found in the soils significantly reduce the mixing of tracers in the subsurface and provide a unique opportunity to examine the role of climate variation on recharge in arid climates. Tracing techniques and core data are examined in this work to reconstruct the paleohydrologic conditions existing in the vadose zone well beyond the timescales typically investigated. Stable chloride and chlorine 36 profiles indicate that the soil waters deep in the vadose zone range in age from approximately 20,000 to 120,000 years. Secondary chloride bulges that are present in two of the three profiles support the concept of recharge occurring at or near the last two glacial maxima, when the climate of the area was considerably wetter and cooler. The stable isotopic composition of the soil water in the profiles is significantly more depleted in heavy isotopes than is modern precipitation, suggesting that recharge under the current climate is not occurring at this arid site. Past and present recharge appears to have been strongly controlled by surface topography, with increased incidence of recharge where runoff from the surrounding mountains may have been concentrated. The data obtained from this detailed drilling and sampling program shed new light on the behavior of water in thick vadose zones and, in particular, show the sensitivity of arid regions to the extreme variations in climate experienced by the region over the last two glacial maxima.

The impact of climate variability on the water cycle in desert ecosystems is controlled by biospheric feedback at interannual to millennial timescales. This paper describes a unique field dataset from weighing lysimeters beneath nonvegetated and vegetated systems that unequivocally demonstrates the role of vegetation dynamics in controlling water cycle response to interannual climate variability related to El Niñ o southern oscillation in the Mojave Desert. Extreme El Niñ o winter precipitation (2.3-2.5 times normal) typical of the U.S. Southwest would be expected to increase groundwater recharge, which is critical for water resources in semiarid and arid regions. However, lysimeter data indicate that rapid increases in vegetation productivity in response to elevated winter precipitation reduced soil water storage to half of that in a nonvegetated lysimeter, thereby precluding deep drainage below the root zone that would otherwise result in groundwater recharge. Vegetation dynamics have been controlling the water cycle in interdrainage desert areas throughout the U.S. Southwest, maintaining dry soil conditions and upward soil water flow since the last glacial period (10,000 -15,000 yr ago), as shown by soil water chloride accumulations. Although measurements are specific to the U.S. Southwest, correlations between satellite-based vegetation productivity and elevated precipitation related to El Niñ o southern oscillation indicate this model may be applicable to desert basins globally. Understanding the two-way coupling between vegetation dynamics and the water cycle is critical for predicting how climate variability influences hydrology and water resources in water-limited landscapes.climate change ͉ ecohydrology ͉ El Niñ o

The quasi-linear pararneterization for unsaturated hydraulic conductivity K(•) = Ks exp (a•), where K is hydraulic conductivity, W is soil water matric potential, Ks is saturated hydraulic conductivity, and a is a porous material parameter, has been used in both stochastic and deterministic models of unsaturated water flow in porous materials. In the stochastic approach, Ks is assumed lognormally distributed, but a and the volumetric soil water capacit•y C = d O/d•, with 0 volumetric soil water content, are assumed normally distributed. We point out here that a and Ks are related to the same internal pore geometry of the soil. This interrelationship ensures that if Ks is lognormal, then a, and possibly C, will also be lognormal. Additionally, we present preliminary field results which indicate that a is better described by a lognormal than a normal distribution. The quasi-linear parameterization can be expected to be correct only in some integral sense. Predictions of increases in the variability of hydraulic conductivity with decreasing ß may therefore be prejudiced by the use of the exponential form for K(•). Tests of the sensitivity of stochastic model predictions to both the parameterizations adopted for K(½) and the assumed distribution functions of parameters seem warranted. Reliable experimental evidence on field variability of K(•) and •(0) at substantial negative values of ß are also needed.

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