Abstract. GlobWat is a freely distributed, global soil water balance model that is used by the Food and Agriculture Organization (FAO) to assess water use in irrigated agriculture, the main factor behind scarcity of freshwater in an increasing number of regions. The model is based on spatially distributed high-resolution data sets that are consistent at global level and calibrated against values for internal renewable water resources, as published in AQUASTAT, the FAO's global information system on water and agriculture. Validation of the model is done against mean annual river basin outflows. The water balance is calculated in two steps: first a "vertical" water balance is calculated that includes evaporation from in situ rainfall ("green" water) and incremental evaporation from irrigated crops. In a second stage, a "horizontal" water balance is calculated to determine discharges from river (sub-)basins, taking into account incremental evaporation from irrigation, open water and wetlands ("blue" water). The paper describes the methodology, input and output data, calibration and validation of the model. The model results are finally compared with other global water balance models to assess levels of accuracy and validity.
Abstract. Water Accounting Plus (WA+) is a framework that summarizes complex hydrological processes and water management issues in river basins. The framework is designed to use satellite-based measurements of land and water variables and processes as input data. A general concern associated with the use of satellite measurements is their accuracy. This study focuses on the impact of the error in remote sensing measurements on water accounting and information provided to policy makers. The Awash Basin in the central Rift Valley in Ethiopia is used as a case study to explore the reliability of WA+ outputs, in the light of input data errors. The Monte Carlo technique was used for stochastic simulation of WA+ outputs over a period of 3 yr. The results show that the stochastic mean of the majority of WA+ parameters and performance indicators are within 5 % deviation from the original WA+ values based on one single calculation. Stochastic computation is proposed as a standard procedure for WA+ water accounting because it provides the uncertainty bandwidth for every WA+ output, which is essential information for sound decision-making processes. The majority of WA+ parameters and performance indicators have a coefficient of variation (CV) of less than 20 %, which implies that they are reliable and provide consistent information on the functioning of the basin. The results of the Awash Basin also indicate that the utilized flow and basin closure fraction (the degree to which available water in a basin is utilized) have a high margin of error and thus a low reliability. As such, the usefulness of them in formulating important policy decisions for the Awash Basin is limited. Other river basins will usually have a more accurate assessment of the discharge in the river mouth.
Abstract. Water Accounting Plus (WA+) is a framework that summarizes complex hydrological processes and water management issues in river basins. The framework is designed to use satellite based measurements of land and water as input data. A concern associated with the use of satellite measurements is their accuracy. This study focuses on the impact of the error in remote sensing measurements on water accounting and information provided to policy makers. The Awash basin in the central rift valley in Ethiopia is used as a case study to explore the reliability of WA+ outputs, in the light of input data errors. The Monte Carlo technique was used for stochastic simulation of WA+ outputs over a period of three years. The results show that the stochastic mean of the majority of WA+ parameters and performance indicators are within 5% deviation from the original values. Stochastic simulation can be used as part of a standard procedure for WA+ water accounting because it provides the error bandwidth for every WA+ output, which is essential information for sound decision making. The majority of WA+ parameters and performance indicators have a Coefficient of Variation (CV) of less than 20% which implies that they are reliable. The results also indicate that the "utilized flow" and "basin closure fraction" (the degree to which available water in a basin is utilized) have a high margin of error and thus a low reliability. As such it is recommended that they are not used to formulate important policy decisions.
Abstract. GlobWat is a freely distributed, global soil water balance model that is used by FAO to assess water use in irrigated agriculture; the main factor behind scarcity of freshwater in an increasing number of regions. The model is based on spatially distributed high resolution datasets that are consistent at global level and calibrated against values for Internal Renewable Water Resources, as published in AQUASTAT, FAO's global information system on water and agriculture. Validation of the model is done against mean annual river basin outflows. The water balance is calculated in two steps: first a "vertical" water balance is calculated that includes evaporation from in situ rainfall ("green" water) and incremental evaporation from irrigated crops. In a second stage, a "horizontal" water balance is calculated to determine discharges from river (sub-)basins, taking into account incremental evaporation from irrigation, open water and wetlands ("blue" water). The paper describes methodology, input and output data, calibration and validation of the model. The model results are finally compared with other global water balance models.
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