Water and salt balances at farmer fields

In this chapter the agrohydrological model SWAP is used to analyse the water flow and salt transport at the measured farmer field

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

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

Assessment of alternative water management options for irrigated agriculture

A simulation study on alternative water management strategies was carried out for Sirsa Irrigation Circle in Haryana, covering an area of about 4800 km(2). Results showed that crop evapotranspiration and soil salinity development under reduction in canal water supply and increase in groundwater use, are largely influenced by the amount and distribution of rainfall. Reduction in canal water supply by 25% during the rainy season is unlikely to have any adverse effect on the salinity development in the study area. Reduction in crop evapotranspiration due to decreased canal water supply can partly be compensated by the increase in groundwater use. Leaching of salts due to monsoon rains in the study area shows that groundwater of even relatively poor quality can be used for irrigation without excessive long-term build up of soil salinity under deep groundwater depth conditions. However, increased groundwater extraction without associated actions will not be very effective to solve the problem of rising groundwater levels

Prediction of Long-Term Drainage-Water Salinity of Pipe Drains

Long term drainage water salinity of pipe drains is modeled with the advection-dispersion equation for the zone above drain level and stream functions for the zone below drain level. Steady-state water flow is assumed. The model is applied to two experimental pipe drainage sites in Haryana State, India. Calculations are conducted for different values of leaching fraction and drain spacing. On average, comparison between measured and predicted drainage water salinity is satisfactory for both the Sampla site and the Hisar site. Calculations show that it may take 15-50 years before drainage water salinity has reduced to equilibrium levels. Leaching fraction has considerable influence on the drainage water salinity. An increase in the leaching fraction from 0.2 to 0.4 will reduce the time to reach equilibrium drainage water salinity levels by about 50%. Drain spacing has little influence on drainage water salinity, provided the hydraulic properties below drain level are uniform (Sampla). Some influence of drain spacing might be expected if the zone below drain level consists of a less conductive layer underlain by a more conductive layer (Hisar). In the latter case, the larger the drain spacing, the longer the time to achieve equilibrium drainage water salinity levels. (C) 2000 Elsevier Science B.V

Effect of Model Selection on Computed Water Balance Components

Soil water flow modelling approaches as used in four selected on-farm water management models, namely CROPWAT. FAIDS, CERES and SWAP, are compared through numerical experiments. The soil water simulation approaches used in the first three models are reformulated to incorporate ail evapotranspiration process similar to that used in SWAP. Computations are carried out for three soil types, representing sandy loam. loam and sandy clay loam The reformulated models are calibrated against simulation results obtained with SWAP. All the modelling approaches predict nearly equal estimates of cumulative actual evapotranspiration for a wheat crop. When compared with SWAP Simulation results, the CERES type approach outperformed the other two approaches in respect of estimated cumulative deep percolation losses. A new criterion is proposed to interpret simulation results under deep water table conditions to suggest appropriate depth of water application. The resulting recommendations for irrigation planning suggest that any of the modelling approaches may be used to suggest practical Irrigation considered in the present study

Water and salt balances at farmer fields

In this chapter the agrohydrological model SWAP is used to analyse the water flow and salt transport at the measured farmer field

Assessment of alternative water management options for irrigated agriculture

A simulation study on alternative water management strategies was carried out for Sirsa Irrigation Circle in Haryana, covering an area of about 4800km2. Results showed that crop evapotranspiration and soil salinity development under reduction in canal water supply and increase in groundwater use, are largely influenced by the amount and distribution of rainfall. Reduction in canal water supply by 25% during the rainy season is unlikely to have any adverse effect on the salinity development in the study area. Reduction in crop evapotranspiration due to decreased canal water supply can partly be compensated by the increase in groundwater use. Leaching of salts due to monsoon rains in the study area shows that groundwater of even relatively poor quality can be used for irrigation without excessive long-term build up of soil salinity under deep groundwater depth conditions. However, increased groundwater extraction without associated actions will not be very effective to solve the problem of rising groundwater levels.Canal water Distributed modeling Evapotranspiration Groundwater Irrigation Soil salinity Water management

Distributed ecohydrological modelling to evaluate irrigation system performance in Sirsa district, India II: Impact of viable water management scenarios

This study focuses on the identification of appropriate strategies to improve water management and productivity in an irrigated area of 4270 km2 in India (Sirsa district). The field scale ecohydrological model SWAP in combination with field experiments, remote sensing and GIS has been applied in a distributed manner generating the required hydrological and biophysical variables to evaluate alternative water management scenarios at different spatial and temporal scales. Simulation results for the period 1991¿2001 show that the water and salt limited crop production is 1.2¿2.0 times higher than the actual recorded crop production. Improved crop husbandry in terms of improved crop varieties, timely sowing, better nutrient supply and more effective weed, pest and disease control, will increase crop yields and water productivity in Sirsa district. The scenario results further showed that reduction of seepage losses to 25¿30% of the total canal inflow and reallocation of 15% canal water inflow from the northern to the central canal commands will improve significantly the long term water productivity, halt the rising and declining groundwater levels, and decrease the salinization in Sirsa district