Estimation of effective soil hydraulic parameters for water management studies in semi-arid zones : integral use of modelling, remote sensing and parameter estimation

Abstract

Key words: evapotranspiration, effective soil hydraulic parameters, remote sensing, regional water management, groundwater use, Bhakra Irrigation System, India.The meaningful application of water management simulation models at regional scale for the analysis of alternate water management scenarios is often hindered by the lack of required input data. Especially information on relevant soil hydraulic parameters is required for the successful application of these models. The focus of this study is the development of techniques to determine effective regional soil hydraulic parameters by making integral use of simulation models, remote sensing information and parameter estimation procedures. The Sirsa Irrigation Circle, covering an area of 0.48 million ha, located in the Bhakra Irrigation system in the North West India is used as a case study.Forwards and backward simulations with the SWAP model for homogeneous soil profiles proved that actual evapotranspiration ( ETa ) rates can be used to inversely identify effective soil hydraulic parameters. ETa rates from fully developed crops during water stress periods are most suitable for this purpose. Frequent measurement on ETa rates is desired not only to precisely estimate the soil hydraulic parameters but also to reduce the undesirable correlation between different fitting parameters.Forwards and backward simulations for seven heterogeneous soil profiles showed that for practical applications, effective soil hydraulic parameters can sufficiently describe the hydraulic behaviour for such profiles. However, if actual soil evaporation and transpiration are to be simulated separately and interpreted independently, an empirical formulation of soil evaporation is very important if the surface soil layers have deviating soil hydraulic properties. The effective soil hydraulic parameters for heterogeneous soils can be determined using information on ETa rates provided general information on soil texture is known.Twenty three NOAA AVHRR satellite images were used to produce ETa maps of the Sirsa Irrigation Circle. The ETa rates were then used to inversely identify the soil hydraulic parameters for the distributed irrigation water management model FRAME. The inversely identified soil hydraulic parameters were in good agreement with the expected values but could not be validated due to absence of such information at the scale of model application.Information on remotely sensed ETa rates and field observation on groundwater levels was also used to estimate actual groundwater use in the study area. Observations on groundwater heads for five years was then used to inversely identify the drainable porosity of the aquifer. Subsequently the model results were validated against observed groundwater heads for nine years and overall good agreement was obtained.After calibration, the FRAME model was used to analyse irrigation water management at regional scale. Three alternate water management scenarios involving partial reduction in canal water supply and increased groundwater extraction were simulated. Effects of alternate water management scenarios on crop ETa and groundwater level depths were predicted. It was observed that in the study area where groundwater levels are rising reduction in canal water supply by 25 % during rainy season is unlikely to have any adverse effect on the development of soil salinity. Reduction in ETa due to decreased canal water supply can be partly compensated for by the increase in groundwater use