Low-streamflow hydrographs from 22 subbasins in the U.S. Department of Agriculture-Agricultural Research Service (USDA-ARS) Washita River Experimental Watershed complex in central Oklahoma were ...subjected to recession slope analysis; this method, after that of Brutsaert and Nieber 1977, was derived from a Dupuit-Boussinesq formulation for the groundwater outflows from the adjoining phreatic aquifers. The long-time aquifer response characteristics were generally found to be close to linear, and the short-time response characteristics were consistent with Boltzmann similarity. Representative values of the resulting basin-scale effective groundwater parameters were (35 days)-1 for the low-flow extinction coefficient (i.e., a storage half-life of 25 days); 0.021 m2 s-1 for the hydraulic diffusivity, Dh; 0.0035 m2 s-1/2 for the hydraulic desorptivity, Deh 8 x 10-4 ms-1 for the hydraulic conductivity k; and 0.018 for the drainable porosity (or specific yield), f. The variabilities of Dh, Deh, and k from basin to basin could be better represented by the log-normal than by the normal distribution; f could be described nearly equally well by both. The storage half-life is moderately and positively correlated with basin size; in the case of k the correlation is negative but weaker. Any scale dependence of Dh, Deh, and f appears to be negligible
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During the First ISLSCP (International Satellite Land Surface Climatology Project) Field experiment (FIFE) the first stage of drying (with the atmosphere controlling) in these natural grasslands ...occurred when the soil was moist with a volumetric soil moisture content SM 27% after rainfall. Both at the local scale and at the regional scale, once SM in the top 10 cm of the soil dropped below about 17% and its vertical gradient started to exceed about 1.1%/cm at 5 cm, the cumulative daily evaporation rate could be taken to be proportional to the square root of time, t1/2, which is not unlike a second stage of drying (with the soil moisture content controlling). Between these two drying stages, there was a transition period which lasted from several days to 2 weeks depending on the soil moisture conditions and on the season. The longer transition periods were observed under conditions of lower net radiation and of higher soil moisture content at depths in excess of, say, 50 cm. The present findings of gradual transitions from the first stage to a desorptive second stage of drying for a surface covered with grass (which can extract water from greater depths) are in contrast to earlier findings of relatively abrupt transitions for bare soil
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Experimental data recorded over a natural tallgrass prairie during the later stages of drying in the First International Satellite Land Surface Climatology Project (ISLSCP) Field Experiment-1987 ...showed (1) that the total daily values of evaporation exhibited a kind of second stage of drying behavior with a t-1/2 dependency at the daily timescale and (2) that this day-to-day evolution was modulated by the available energy at the surface, that is, the hourly radiation input. This allowed a simple description of the phenomenon by combining a desorptive diffusion-type parameterization for the total daily evaporation or for its dimensionless counterpart (such as Priestley and Taylor's alpha, the evaporative fraction, and a few others), with an assumption of self-preservation in the surface energy budget during the daytime hours. The resulting formulation, which involves two timescales, a daily and an hourly, was able to reproduce daytime hourly flux values over a 2-week period of intensive drying. The method can also be useful in the disaggregation of daily, or even weekly, evaporation into hourly values
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The fifth intensive field campaign of FIFE (First ISLSCP Field Experiment (International Satellite Land Surface Climatology Project)) was conducted in northeastern Kansas during late summer 1989, in ...order to study the dry down of a surface with prairie vegetation prior to senescence and dormancy. As precipitation became sparse and rare, the surface developed nonuniform features at the scales of the rainfall events. Nevertheless, lower boundary layer measurements used together with remotely sensed surface temperatures on the basis of Monin-Obukhov similarity allowed the estimation of the flux of sensible heat for a region with characteristic dimensions of 10(4) M. In contrast, under these conditions it was difficult to aggregate an array of local flux measurements into a true regional mean. On account of the same problem in the aggregation of local measurements of net radiation and ground heat flux, the reliability of the estimation of regional evaporation by the energy budget method was hard to assess.
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Radiosonde measurements above the heterogeneous forest of the Landes region in southwestern France provided vertical profiles of potential temperature and specific humidity in the atmospheric ...boundary layer. For all of the 62 profiles analyzed under unstable atmospheric conditions a surface sublayer could be identified within which the Monin-Obukhov similarity was consistent with the regional surface fluxes of sensible heat and latent heat (evaporation). For the potential temperature the vertical extent of this sublayer was found to be 41 (+/- 30) less than or equal to (z - do)/zo less than 130 (+/- 49), where zo = 1.2 m is the roughness height and do = 6.0 m is the displacement height; for the specific humidity it was 48 (+/- 36) less than or equal to (z - do)/zo less than or equal to 153 (+/- 63). These results show that irregular forest surfaces are not anomalous, as regards the lower limit of the surface layer in comparison with surfaces with smaller roughness. The surface fluxes derived from the profile measurements were compared with flux measurements obtained by means of an eddy correlation system atop a 29 m mast, some 9 m above a mature section of forest some 4.5 km away from the launching site of the radiosondes; these independent measurements were made by a team from the Institute of Hydrology (Wallingford, England). On average, these two types of estimates were in good agreement. For the sensible heat flux the correlation coefficient was r = 0.75. For the evaporation rate it was r = 0.66. For the evaporation obtained by means of the energy budget from the sensible beat flux it improved to r = 0.82. The profile-derived fluxes did not compare as favorably with corresponding flux values measured above agricultural crops in clearings. This confirms that the forest was the dominant surface at the regional scale
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