Wilfried Brutsaert

Regionalized drought flow hydrographs from a mature glaciated plateau

The drought or base flow characteristics of six basins in the Finger Lakes region are obtained by considering for each available record the lower envelope of |dQ/dt| as a function of Q, where Q is the flow rate. This procedure avoids the uncertainty regarding a proper time reference after each rainfall event, and it eliminates the effects of evapotranspiration. The results suggest that among several expressions, Boussinesqs nonlinear solution of free surface groundwater flow is best suited to parameterize the observed hydrographs. The obtained parameters can be related to the basin characteristics, viz., drainage area and the total stream length, in accordance with relationships derived on the basis of the Dupuit-Boussinesq aquifer model. This result allows the determination of drought flow parameters for ungaged sites within the region.

Hydrologic Cycle explains the evaporation paradox

The evaporation of water, measured using evaporation pans, has been decreasing in the past few decades over large areas with different climates. The common interpretation is that the trend is related to increasing cloudiness, and that it provides an indication of decreasing potential evaporation and a decreasing terrestrial evaporation component in the hydrologic cycle. Here we show that, although these studies are valuable, pan evaporation has not been used correctly as an indicator of climate change.

Effective water table depth to describe initial conditions prior to storm rainfall in humid regions

A physically meaningful technique to determine the effective depth to the water table, as a measure of the initial storage capacity of a basin is developed. The estimation of the initial storage capacity prior to a given flood event is essential to obtain useful results from storm runoff prediction models based on saturation excess overland flow. It is shown how this effective depth to the water table can be related to streamflow measurements at the outlet of the basin. The analysis is based on Boussinesqs standard hydraulic groundwater theory. The main feature of the present formulation is that it allows the estimation of catchment-scale parameters, namely the aquifer hydraulic conductivity and the average depth to the impervious layer. The estimation of these parameters is based on a drought flow analysis which is consistent with the hydraulic groundwater theory used to develop the described technique. This hydraulic theory is found to be applicable for a catchment under humid temperate climatic conditions, namely the Zwalm catchment situated in East-Flanders, Belgium. The results of the proposed analysis are used to estimate the initial conditions in a partial area runoff generation model. It is shown that accurate estimates of total runoff volume are obtained without further calibration of the model.

Application of self-preservation in the diurnal evolution of the surface energy budget to determine daily evaporation

Evaporation from natural land surfaces often exhibits a strong variation during the course of a day, mostly in response to the daily variation of radiative energy input at the surface. This makes it difficult to derive the total daily evaporation, when only one or a few instantaneous estimates of evaporation are available. It is often possible to resolve this difficulty by assuming self-preservation in the diurnal evolution of the surface energy budget. Thus if the relative partition of total incoming energy flux among the different components remains the same, the ratio of latent heat flux (LE) and any other flux component can be taken as constant through the day. This concept of constant flux ratios is tested by means of data obtained during the First ISLSCP Field Experiment (FIFE); the instantaneous evaporation values were calculated by means of the atmospheric boundary layer (ABL) bulk similarity approach with radiosonde profiles and radiative surface temperatures. Good results were obtained for evaporative flux ratios with available energy flux (LE/(Rn - G)), with net radiation (LE/Rn), and with incoming shortwave radiation (LE/S ↓).

THE UNIT RESPONSE OF GROUNDWATER OUTFLOW FROM A HILLSLOPE

Subsurface flow from a hillslope can be described by hydraulic groundwater theory, as formulated by the Boussinesq equation. Solution of its linear version brings out some of the essential features of this phenomenon. Arbitrary inputs can be accommodated by simple convolution of the sudden drawdown problem. The relative magnitude of the driving mechanisms, namely the streamwise pressure gradient, resulting in diffusion and gravity (i.e., slope), resulting in advection, can be determined by a simple dimensionless parameter.

BASIN-SCALE GEOHYDROLOGIC DROUGHT FLOW FEATURES OF RIPARIAN AQUIFERS IN THE SOUTHERN GREAT PLAINS

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 longtime 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 m2s−½ for the hydraulic desorptivity, Deh; 8 × 10−4 ms−1 for the hydraulic conductivity k; and 0.018 for the drainable porosity (or specific yield), ƒ. The variabilities of Dh, Deh, and k from basin to basin could be better represented by the log-normal than by the normal distribution; ƒ 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 ƒ appears to be negligible.

Daytime evaporation and the self-preservation of the evaporative fraction and the Bowen ratio

Abstract Under certain conditions several ratios of fluxes in the surface energy budget are known to remain relatively constant during the daytime hours. This ‘self-preservation’ has allowed the use of midday values of the evaporative fraction, EF, multiplied by the daytime total of net radiation minus the ground heat flux, to provide a good estimate of total daytime evaporation. A similar method is discussed, in which the daytime total sensible heat flux is divided by the noon Bowen ratio (β = (1/EF) - 1). Data from the First International Satellite Land Surface Climatology Project (ISLSCP) Field Experiment (FIFE) indicate that the two methods give rather different results. It is shown by propagating uncertainties that the uncertainty in the daytime total evaporation is likely to be smaller with the constant-EF method than with the constant-β method for β greater than about - 1/2.

Aspects of bulk atmospheric boundary layer similarity under free‐convective conditions

For many hydrologic and atmospheric dynamic purposes the turbulent surface fluxes of sensible and latent (or evaporative) energy and of momentum must be formulated over areas covering a wide range of spatial scales; these are characteristically the scales of river basins and of the grid sizes for integration in current atmospheric circulation models. The bulk atmospheric boundary layer (ABL) similarity (BAS) approach continues to be one of the very few available formulations to describe turbulent surface fluxes over the range of scales from roughly 1 to 10 km, in terms of average characteristics aloft in the upper reaches of the ABL. The approach is reexamined as it relates to atmospheric stability and surface characteristics under conditions of free convection. Since the largest gradients occur near the surface, first a formulation is proposed for the Monin-Obukhov profile functions for convective conditions that is consistent with the theoretical advances by Kader and Yaglom [1990] and with recent experimental data. This is then combined with a slab representation of the mixed layer to derive the most plausible form of the BAS functions, on the basis of presently available information. Finally, it is illustrated with available experimental data that any alternative formulation of bulk ABL transport, regardless of its appearance, is of necessity equivalent with BAS and that the same original variables are at work.

Daily evaporation over a region from lower boundary layer profiles measured with radiosondes

Regional daily evaporation was estimated by means of continuous measurements of the available energy flux at the surface and one or more instantaneous determinations of the evaporative fraction (EF). The data were obtained during the First ISLSCP Field Experiment (FIFE) in northeastern Kansas. EF, which is the ratio of the latent heat flux and the available energy flux, was assumed to be constant during the daylight hours; thus it was determined from only a few instantaneous surface flux values calculated on the basis of radiosonde profiles in the surface layer, together with remotely sensed surface temperature. Comparison of 23 estimated and measured daytime evaporation values showed good correlation (r = 0.97), although the evaporation was underestimated by about 5% on average. In the estimation of daily evaporation, nighttime evaporation must also be considered; measurements by means of the eddy correlation method showed that on average it accounted for some 8% of the total daily evaporation.

Heat and mass transfer to and from surfaces with dense vegetation or similar permeable roughness

A differential equation is obtained to describe the concentration of passive admixtures (water vapor, sensible heat, pollutants, CO2, etc.) of turbulent flow inside a dense and uniform vegetational canopy. The profiles of eddy diffusivity, wind speed and shear stress are assumed to be exponential decay functions of depth below the top of the canopy. This equation is solved for the case of a vegetation with constant concentration of the admixture at the foliage surfaces. The solution is used to formulate bulk mass or heat transfer coefficients, which can be applied to practical problems involving surfaces covered with a vegetation or with similar porous or fibrous roughness elements. The results are shown to be consistent with experimental data presented by Chamberlain (1966), Garratt and Hicks (1973) and Garratt (1978). Calculations with the model illustrate that, as compared to its behavior over surfaces with bluff roughness elements, ln(z0/z0c) (wherez0 is the momentum roughness andz0c, the scalar roughness) for permeable roughness elements is relatively insensitive tou* and practically independent ofz0.