R.A. Feddes

A remote sensing surface energy balance algorithm for land (SEBAL); 1 formulation

The major bottlenecks of existing algorithms to estimate the spatially distributed surface energy balance in composite terrain by means of remote sensing data are briefly summarised. The relationship between visible and thermal infrared spectral radiances of areas with a sufficiently large hydrological contrast (dry and wet land surface types, vegetative cover is not essential) constitute the basis for the formulation of the new Surface Energy Balance Algorithm for Land (SEBAL). The new algorithm (i) estimates the spatial variation of most essential hydro-meteorological parameters empirically, (ii) requires only field information on short wave atmospheric transmittance, surface temperature and vegetation height, (iii) does not involve numerical simulation models, (iv) calculates the fluxes independently from land cover and (v) can handle thermal infrared images at resolutions between a few meters to a few kilometers. The empirical relationships are adjusted to different geographical regions and time of image acquisition. Actual satellite data is inserted in the derivation of the regression coefficients. Part 2 deals with the validation of SEBAL. q 1998 Elsevier Science BV. All rights reserved.

Simulation model of the water balance of a cropped soil: SWATRE

Abstract A transient one-dimensional finite-difference model for the unsaturated zone with water uptake by roots is presented. A number of boundary conditions is given for the top and the bottom of the system. At the top, 24-hr. data on rainfall, potential soil evaporation and potential transpiration are needed. When the soil system remains unsaturated, one of three bottom boundary conditions can be used: pressure head, zero flux or free drainage. When the lower part of the system remains saturated, one can either give the groundwater level or the flux through the bottom of the system as input. In the latter case the groundwater level is computed.

Simulation of field water uptake by plants using a soil water dependent root extraction function

Abstract Water uptake by roots can be represented by adding a volumetric sink term to the continuity equation for soil-water flow. This sink term is often expressed as a product of the difference in pressure head between the soil and the root-soil interface, the hydraulic conductivity of the soil and some empirical root function. Because of the amount of field work and experimental difficulties involved in determining this root function, an attempt was made to describe the sink term with a more simple expression. In this approach the sink term is considered to be a function of the soil-water content, varying with the latter according to the pressure heads generally known to be critical for water uptake by the roots. An implicit finite-difference model was developed and verified with results obtained experimentally in the field from water-balance studies. Although the model does not predict the distribution of soil-water content with depth in very accurate detail, the cumulative effect over the entire depth is properly simulated. Comparison of the results of the simpler model with those of the model of Feddes, Bresler and Neuman, leads to the conclusion that both models yield comparable results.

The scaling characteristics of soil parameters: From plot scale heterogeneity to subgrid parameterization

Abstract The variation in soil texture, surface moisture or vertical soil moisture gradient in larger scale atmospheric models may lead to significant variations in simulated surface fluxes of water and heat. The parameterization of soil moisture fluxes at spatial scales compatible with the grid size of distributed hydrological models and mesoscale atmospheric models (∼ 100 km2) faces principal problems which relate to the underlying microscopic or field scale heterogeneity in soil characteristics. The most widely used parameterization in soil hydrology, the Darcy-Richards (DR) equation, is gaining increasing importance in mesoscale and climate modelling. This is mainly due to the need to introduce plant-interactive soil water depletion and stomatal conductance parameterizations and to improve the calculation of deep percolation and runoff. Covering a grid of several hundreds of square kilometres, the DR parameterization in soil-vegetation-atmosphere-transfer schemes (SVATs) is assumed to be scale-invariant. The parameters describing the non-linear, area-average soil hydraulic functions in this scale-invariant DR-equation should be treated as calibration-parameters, which do not necessarily have a physical meaning. The saturated hydraulic conductivity is one of the soil parameters to which the models show very high sensitivity. It is shown that saturated hydraulic conductivity can be scaled in both vertical and horizontal directions for large flow domains. In this paper, a distinction is made between effective and aggregated soil parameters. Effective parameters are defined as area-average values or distributions over a domain with a single, distinct textural soil type. They can be obtained by scaling or inverse modelling. Aggregated soil parameters represent grid-domains with several textural soil types. In soil science dimensional methods have been developed to scale up soil hydraulic characteristics. With some specific assumptions, these techniques can be extrapolated from classical field-scale problems in soil heterogeneity to larger domains, compatible with the grid-size of large scale models. Particularly promising is the estimation of effective soil hydraulic parameters from area averaging measurements through inverse modelling of the unsaturated flow. Techniques to scale and aggregate the soil characteristics presented in this paper qualify for direct or indirect use in large scale meteorological models. One of the interesting results is the effective behaviour of the reference curve, which can be obtained from similar media scaling. If the conclusions of this paper survive further studies, a relatively simple method will become available to parameterize soil variability at large scales. The inverse technique is found to provide effective soil parameters which perform well in predicting both the area-average evaporation and the area-average soil moisture fluxes, such as subsurface runoff. This is not the case for aggregated soil parameters. Obtained from regression relationships between soil textural composition and hydraulic characteristics, these aggregated parameters predict evaporation fluxes well, but fail to predict water balance terms such as percolation and runoff. This is a serious drawback which could eventually hamper the improvement of the representation of the hydrological cycle in mesoscale atmospheric models and in GCMs.

Is large-scale inverse modelling of unsaturated flow with areal average evaporation and surface soil moisture as estimated from remote sensing feasible?

Abstract The potentiality of combining large-scale inverse modelling of unsaturated flow with remote sensing determination of areal evaporation and areal surface moisture is assessed. Regional latent and sensible heat fluxes are estimated indirectly using remotely sensed measurements by parameterizing the surface energy balance equation. An example of evapotranspiration mapping from northern and central Egypt is presented. The inverse problem is formulated with respect to the type of information available. Two examples of estimation of soil hydraulic properties by the dynamic one-dimensional soil-water-vegetation model SWATRE are given: one refers to a classical lysimeter scale and another one to a catchment scale. It is concluded that small-scale soil physics may describe large-scale hydrological behaviour adequately, and that the effective hydraulic parameters concerned may be derived by an inverse modelling approach. Remotely sensed data on surface reflectance, surface temperature and soil moisture content derived from multifrequency microwave techniques provide a useful data set on the mesoscale. The inverse modelling approach presented combined with a meso-scale data set on evaporation and surface soil moisture, considerable potentialities arise to determine effective meso-scale hydraulic properties.

Sustainability of irrigated agriculture.

Preface. Research Agenda L.S. Pereira, et al. Part I: Sustainability Concerns in Irrigated Agriculture. Irrigated Agriculture at the Crossroads M.E. Jensen. Economics of Irrigation I. Carruthers. Institutional Questions and Social Challenges H. Hill, L. Tollefson. Health Impacts of Agricultural Development I. Hespanol. Vulnerability of Soils under Irrigation J. Porta, J. Herrero. Sustainability Concerns of Irrigated Agriculture L.K. Smedema. Part II: Soil and Water Conservation and Water Harvesting (Or Rainfed Systems). Sustainability of Soil and Water Conservation in Sub-Saharan Africa C. Reij, W. Critchley. Soil and Water Conservation in Tunisia H. Missaoui. Water Harvesting - Past and Future D. Prinz. Part III: On-Farm Water Management. Measurement and Estimation of Evapotranspiration B. Itier. Water Use Efficiency P. Steduto. Modeling of Water Flow and Solute Transport for Irrigation and Drainage J.C. van Dam, R.A. Feddes. Irrigation Scheduling D.F. Heermann. Irrigation Scheduling in the Agronomic Practice A. Yazar, et al. Part IV: On-Farm Irrigation and Drainage Systems. Surface Irrigation Systems L.S. Pereira. Sprinkler Irrigation Systems J.R. Gilley. Micro-Irrigation Systems and Fertigation I. Papadopoulos. Drainage of Irrigated Land S. Bouarfa, et al. Part V: Water Quality Management. Salinity Management in Irrigated Agriculture N.K. Tyagi. Use and Management of Saline Water for Irrigation Towards Sustainable Development A. Hamdy. Agrochemicals and Water Management R.S. Kanwar. Water and Nitrate Balance in Irrigated Soils G. Vachaud, et al. Nitrate Leaching Under Irrigated Agriculture F. Moreno, et al. Waste-water Reuse S. Kyritsis. Part VI: Irrigation Scheme Management. Sustainability Concerns in the Operation and Maintenance of Irrigation Systems J.A. Sagardoy. Performance Parameters for a Decentralized and Participatory Water Administration J. Chambouleyron. Remote Sensing, GIS and Hydrological Modelling for Irrigation Management M. Menenti, et al. Regulation and Control in Irrigation Systems J. Goussard. Remote Control and Management of Irrigation Delivery Systems P. Kosuth. Part VII: Capacity Building. Role of Consulting Services J. Hennessy. Professional Training Requirements J. Feyen. North-South Cooperative Research on Sustainability of Water Resources Utilization in Agriculture M. Catizzone. Technology Transfer for Sustainable Water Resources Development M. Smith. Part VIII: Regional Perspectives. Sustainability Concerns in Asian Irrigation K. Mohtadullah, et al. Sustaining Irrigated Agriculture in China L. Cai, et al. Sustainability Concerns in African Irrigation F.N. Gichuki. Assessment of Impacts of Irrigated Agriculture: A Case Study A. Poulovassilis, et al. Annex I: Format of the Workshop. Annex II: List of Papers Presented to the Workshop. Annex III: List of Participants.

Simulation of root water uptake: II. Non-uniform transient water stress using different reduction functions

The macroscopic root water uptake approach was used in the numerical simulation model HYSWASOR to test four different pressure head-dependent reduction functions. The input parameter values were obtained from the literature and derived from extensive measurements under controlled conditions in the greenhouse. The simulation results indicated that the linear reduction function cannot fit the data satisfactorily. Most of the existing non-linear reduction functions can fit only half of the data range, while the best agreement is obtained with the non-linear two-threshold reduction function. The parameter values obtained by calibration differ only slightly from those of the experiments. Soil water pressure head heterogeneity over the root zone does not play an important role in water uptake. The roots appear to take up water from the relatively wetter parts of the root zone to compensate for the water deficit in the drier parts. While the simulated transpiration agrees closely with the experimental data, the main reason for the discrepancy between the simulated and actual water contents appears to be water uptake during the night.

Simulation of root water uptake: I. Non-uniform transient salinity using different macroscopic reduction functions

A macroscopic root extraction model was used with four different reduction functions for salinity stress in the numerical simulation model HYSWASOR. Most of the parameter values originally proposed for these functions did not provide good agreement with the experimental data. Therefore, the parameter values were derived from extensive measurements of one of five salinity treatments of alfalfa experiments in the greenhouse and then validated with the four remaining treatments. The simulation results indicated that a well-known crop yield response function can be used as a water uptake term, using the same crop-specific slope and a modified salinity threshold value. The most sensitive part of this reduction function appeared to be the threshold value; while for the non-linear reduction function, without a threshold, the major sensitivity lies in its shape parameter. The simulated actual cumulative transpirations are rather close to the experimental values, while the simulated soil water contents and soil solution osmotic heads indicate some discrepancies with the actual data, but the mean values of these variables are very close to the measured data. While the non-linear two-threshold reduction function provides better agreement with the experimental data for most treatments, all other functions provided close results. This observation suggests the use of the simple linear reduction function in simulation models.

Assimilation of satellite data into agrohydrological models to improve crop yield forecasts

This paper addresses the question of whether data assimilation of remotely sensed leaf area index and/or relative evapotranspiration estimates can be used to forecast total wheat production as an indicator of agricultural drought. A series of low to moderate resolution MODIS satellite data of the Borkhar district, Isfahan (Iran) was converted into both leaf area index and relative evapotranspiration using a land surface energy algorithm for the year 2005. An agrohydrological model was then implemented in a distributed manner using spatial information of soil types, land use, groundwater and irrigation on a raster basis with a grid size of 250 m, i.e. moderate resolution. A constant gain Kalman filter data assimilation algorithm was used for each data series to correct the internal variables of the distributed model whenever remotely sensed data were available. Predictions for 1 month in advance using simulations with assimilation at a regional scale were very promising with respect to the statistical data (bias = ±10%). However, longer‐term predictions, i.e. 2 months in advance, resulted in a higher bias between the simulated and statistical data. The introduced methodology can be used as a reliable tool for assessing the impacts of droughts in semi‐arid regions.

Analysis of the land surface heterogeneity and its impact on atmospheric variables and the aerodynamic and thermodynamic roughness lengths

transfer kB � 1 . First, in this study the land surface heterogeneity has been documented through the comparison of surface reflectance r0, surface temperature T0, net radiation flux Rn, and sensible heat flux H partitioning over the different land cover types in the experimental areas of the Global Energy and Water Cycle Experiment (GEWEX) Asian Monsoon Experiment on the Tibetan Plateau (GAME/Tibet), the Coordinated Enhanced Observing Period (CEOP) Asia-Australia Monsoon Project on the Tibetan Plateau (CAMP/Tibet), the Heihe Basin Field Experiment (HEIFE), the Arid Environment Comprehensive Monitoring Plan, 95 (AECMP’95), and the Dun Huang Experiment (DHEX). The results show that the surface heterogeneity was very significant in the areas of the HEIFE, the AECMP’95, and the DHEX and that it was less significant in the areas of CAMP/Tibet and GAME/Tibet. Second, the vertical profiles of Ta, u, and q in the near-surface layer and above the blending height zb have been analyzed using the atmospheric boundary layer (ABL) tower data, radiosonde data, and tethered balloon data observed during the HEIFE, the DHEX, and the CAMP/Tibet. The results show that the land surface heterogeneity leads in the near-surface layer to different vertical profiles of u, Ta, and q overlying the surfaces of the Gobi and the oasis in the areas of the HEIFE and DHEX. The values of u, Ta, and q become well mixed above a height of about 300 m at the HEIFE and 150 m at the DHEX. z0m, z0h, and kB � 1 over the different land surfaces have also been determined in this study. The results show that the land surface heterogeneity leads to different aerodynamic and thermodynamic parameters over the areas of the HEIFE, the AECMP’95, and the GAME/Tibet. Citation: Ma, Y., M. Menenti, R. Feddes, and J. Wang (2008), Analysis of the land surface heterogeneity and its impact on atmospheric variables and the aerodynamic and thermodynamic roughness lengths, J. Geophys. Res., 113, D08113,