A variety of aircraft remotely sensed and conventional ground-based measurements of volumetric soil water content (SW) were made over two subwatersheds (4.4 and 631 ha) of the U.S. Department of Agriculture's Agricultural Research Service Walnut Gulch experimental watershed during the 1990 monsoon season. Spatially distributed soil water contents estimated remotely from the NASA push broom microwave radiometer (PBMR), an Institute of Radioengineering and Electronics (IRE) multifrequency radiometer, and three ground-based point methods were used to define prestorm initial SW for a distributed rainfall-runoff model (KINEROS; Woolhiser et al., 1990)•at a small catchment scale (4.4 ha). At a medium catchment scale (631 ha or 6.31 km •) spatially distributed PBMR SW data were aggregated via stream order reduction. The impacts of the various spatial averages of SW on runoff simulations are discussed and are compared to runoff simulations using $W estimates derived from a simple daily water balance model. It was found that at the small catchment scale the SW data obtained from any of the measurement methods could be used to obtain reasonable runoff predictions. At the medium catchment scale, a basin-wide remotely sensed average of initial water content was sufficient for runoff simulations. This has important implications for the possible use of satellite-based microwave soil moisture data to define prestorm SW because the low spatial resolutions of such sensors may not seriously impact runoff simulations under the conditions examined. However, at both the small and medium basin scale, adequate resources must be devoted to proper definition of the input rainfall to achieve reasonable runoff simulations. 1. Introduction Passive microwave soil moisture research has focused on the basic questions involved in the data interpretation algorithm [Jackson and Schmugge, 1989]. There have been a number of efforts to develop water balance models that utilize these surface observations [Jackson, 1986; Prevot et al., 1984]; however, these have only considered a single profile and have not considered surface runoff dynamics. Engrnan and Gurney [1991] recently summarized some common viewpoints concerning remotely sensed soil moisture observations and hydrologic modeling. The general conclusion was that in order to fully utilize the information that frequent spatially distributed soil moisture observations might provide, we must reevaluate the hydrologic models themselves. The soil component of many existing models is constructed in such a way to make the model work even though soil moisture has never been available as an input variable. This element of the hydrologic cycle has thus Gurney [1991] noted, actual observations of soil moisture may offer no improvement in runoff estimation because these models do not properly incorporate this variable. This was observed by Jackson et al. [1981] in a study involving a continuous runoff simulation model. In that study they examined how repetitive surface soil moisture observations could be us...

Measurements of the microwave brightness temperature (TB) with the Pushbroom Microwave Radiometer (PBMR) over the Walnut Gulch Experimental Watershed were made on selected days during the MONSOON 90 field campaign. The PBMR is an L-band instrument (21-cm wavelength) that can provide estimates of nearsurface soil moisture over a variety of surfaces. Aircraft observations in the visible and near-infrared wavelengths collected on selected days also were used to compute a vegetation index. Continuous micromctcorological measurements and daily soil moisture samples were obtained at eight locations during the experimental period. Two sites were instrumented with time domain refleciometry probes to monitor the soil moisture profile. The fraction of available energy used for cvapotranspiraiion was computed by taking the ratio of latent heat flux (LE) to the sum of net radiation (Rn) and soil heat flux (6). This ratio is commonly called the evaporative fraction (EF) and normally varies between 0 and 1 under daytime convective conditions with minimal advection. A wide range of environmental conditions existed during the field campaign, resulting in average EF values for the study area varying from 0.4 to 0.8 and values of TB ranging from 220 to 280 K. Comparison between measured TB and EF for the eight locations showed an inverse relationship with a significant correlation (r2 = 0.69). Other days were included in the analysis by estimating TB with the soil moisture data. Because tran spiration from the vegetation is more strongly coupled to root zone soil moisture, significant scatter in this relationship existed at high values of TB or dry near-surface soil moisture conditions. It caused a substantial reduction in the correlation with r2 = 0.40 or only 40% of the variation in EF being explained by TB. The variation in EF under dry near-surface soil moisture conditions was correlated to the amount of vegetation cover estimated with a remotely sensed vegetation index. These findings indicate that information obtained from optical and microwave data can be used for quantifying the energy balance of semiarid areas. The microwave data can indicate when soil evaporation is significantly contributing to EF. while the optical data is helpful for quantifying the spatial variation in EF due to the distribution of vegetation cover. * Range in midday evaporative fraction from the eight METFLUX sites. ** Northernmost and southernmost flight lines only.

Zones of total stress tension and steep stress gradient occur in soil structures such as multilayer pavements and earth dams, and direct tension tests and simple bending tests on soils could be used advantageously to investigate soil behaviour under these conditions. The Paper describes the observed behaviour in direct tension and bending of two clays compacted with standard Proctor effort at different moisture contents ranging from below to above optimum. The investigation is in terms of total stress and is concerned with both stress-strain behaviour and strength condition. It is shown that the elastic bending theory is not satisfactory for analysing bending in soils and an alternative differential method of analysis is used which does not require the assumption of any preferred stress- strain laws. The maximum tensile strains at failure are observed to increase with the increase of moulding moisture content irrespective of the type of test or the optimum moisture content. This supports the view that clays should be compacted at high moisture contents when considerable tensile deformation of a soil structure is expected. The values of both the initial tangent modulus and of the secant modulus (at maximum stress) for both clays are markedly greater in tension than in compression, and the relative values are influenced by the moulding moisture content. Thus the use of identical values for tension and compression in analysis of soil structure is not justified. Des zones de contrainte de tension totale et de gradient élevé se produisent dans des structures de sol telles que les multicouches et les barrages en terre. Des essais de tension directe et de simples essais de flexion sur les sols pourraient être utilisés avantageusement afin d'étudier le comportement du sol dans ces conditions. L'étude décrit le comportement observé dans une tension non drainée et une flexion de deux argiles compactées au Proctor standard à diverses teneurs en eau inférieures. et supérieures à l'optimum. L'étude est en termes de contraintes totales et elle est concernée par le comportement de contrainte de tension ainsi que par les conditions de résistance. II est montré que la théorie de la flexion élastique n'est pas satisfaisante pour analyser la flexion dans les sols, et une autre méthode différente d'analyse est utilisée qui n'exige pas l'hypothese d'aucune loi spéciale de contrainte déformation. On observe que les déformations maximum de traction à la rupture augmentent avec l'accroissement de la teneur en eau, indépendamment du genre d'essai ou de la teneur optimum en eau. Cela confirme que les argiles devraient être compactées à des teneurs en eau élevées lorsque l'on s'attend à une déformation de traction considérable du sol. Les valeurs du module initial tangent et du module sécant (à contrainte maximum) pour chacune des argiles sont, d'une façon marquée, plus grandes en tension qu'en compression, et les valeurs relatives sont influencées par la teneur en eau. Ainsi, l'utilisation de valeurs identiques en traction et en compression dans l'analyse de la structure du sol n'est pas justifiée.

The push broom microwave radiometer (PBMR) was flown on six flights of the NASA C-130 to map the surface soil moisture over the U.S. Department of Agri.culture's Agricultural Research Service Walnut Gulch experimental watershed in southeastern Arizona. The PBMR operates at a wavelength of 21 cm and has four horizontally polarized beams which cover a swath of 1.2 times the aircraft altitude. By flyifig a series of parallel flight lines it was possible to map the microwave brightness temperature (Ts), and thus the soil moisture, over a large area. In this case the area was approximately 8 by 20 km. The moisture conditions ranged from very dry, •2%by volume, to quite wet, • 15%, after a heavy rain. The rain amounts ranged from less than 10 mm to more than 50 mm over the area mapped with the PBMR. With the PBMR we were able to observe the spatial variations of the rain amounts and the temporal variation as the soil dried. The T• values were registered to a Universal Transverse Mercator grid so that they could be compared to the rain gage readings and to the ground measurements of soil moisture in the 0-to 5-cm layer. The decreases in Ts were well correlated with the rainfall amounts, R 2 = 0.9, and the comparison of T• with soil moisture was also good with an R 2 of about 0.8. For the latter, there was some dependence of the relation on location, which may be due to soil or vegetation variations over the area mapped. The application of these data to runoff forecasts and flux estimates will be discussed.

Synopsis A series of drained and undrained, compression and extension tests have been carried out on Weald Clay, using the triaxial cell. The soil was remoulded at a high water content and normally consolidated or overconsolidated before testing. It is shown that the stress-strain and volume-change characteristics depend upon the overconsolidation ratio, the nature of the external stress change, and the drainage conditions. The effective stress Mohr failure envelopes and Hvorslev parameters are, for practical purposes, independent of the type of test. A limited number of drained and undrained compression tests on London Clay showed, in general, similar behaviour to compression tests on Weald Clay. On a fait une série d'essais drainé et à teneur en eau constante, de compression et de dilatation sur de l'argile du Weald en utilisant la cellule triaxiale. On a conditionné le sol à une teneur en eau constante élévée, et on l'a consolidé et surconsolidé avant les essais. On s'apperçoit que les caractéristiques de tension-déformation et de changement volumétrique dépendent de l'indice de surconsolidation, de la nature du changement de tension externe et des conditions de drainage. Les enveloppes de tension de rupture Mohr effective et les paramétres Hvorslev sont, au point de vue pratique, indépendant du genre d'essai. Un nombre limité d'essais de compression drainés et à teneur en eau constante sur de l'argile londonnienne ont montré, en général une maniére de se comporter semblable à celle des essais sur l'argile de la Weald