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Title: Effects of Estimating Soil Hydraulic Properties and Root Growth Factor on Soil Water Balance and Crop Production

item Ma, Liwang
item Bartling, Patricia
item Ahuja, Lajpat
item Green, Timothy

Submitted to: Agronomy Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/12/2009
Publication Date: 4/27/2009
Citation: Ma, L., Hoogenboom, G., Saseendran, S.A., Bartling, P.N., Ahuja, L.R., Green, T.R. 2009. Effects of Estimating Soil Hydraulic Properties and Root Growth Factor on Soil Water Balance and Crop Production. Agronomy Journal. 101(3) :572-583.

Interpretive Summary: Accurate simulation of plant growth depends not only on plant parameters, but also on soil parameters. Although there is uncertainty in measured soil parameters and root distributions, their effects on simulated plant growth have been much less studied. This study was to evaluate the simulated response of six crops, i.e., wheat (Triticum aestivum L.), maize (Zea mays L.), barley (Hordeum vulgare L.), soybean (Glycine max L. Merrill), peanut (Arachis hypogaea L.), and chickpea (Cicer arietinum L.), under various water and N management to different methods of estimating soil hydraulic properties and soil root growth factor (SRGF) in RZWQM2 (Root Zone Water Quality Model) that contains the DSSAT Version 4.0 (Decision Support System for Agrotechnology Transfer) plant growth models. The two methods of obtaining the soil water retention curve (SWRC) in RZWQM2 were based on (1) known soil water contents at both 33 kPa and 1500 kPa suctions, or (2) soil water content at 33 kPa only. The two methods of estimating saturated hydraulic conductivity (Ksat) were (1) soil texture class based average Ksat or (2) Ksat calculated from effective porosity (difference between soil water contents at saturation and at 33 kPa). For the six crops, simulation results showed that the soil water balance was affected more by Ksat than by SWRC, whereas the simulated crop growth was affected by both Ksat and SWRC. The results also showed that small variation in the SRGF did not affect soil and crop simulations and SRGF could be estimated with a simple exponential equation.

Technical Abstract: Increasing water use efficiency (WUE) is one of the oldest goals in agricultural sciences, yet it is still not fully understood and achieved due to the complexity of soil-weather-management interactions. System models that quantify these interactions are increasingly used for optimizing crop WUE, especially under limited water conditions. In this study, crop production and WUE were investigated with DSSAT4.0 (Decision Support System for Agrotechnology Transfer) model and the RZWQM2 (Root Zone Water Quality Model)-DSSAT4.0 hybrid model for six crops; namely wheat, maize, barley, soybean, peanut, and chickpea. Simulation results showed that both DSSAT4.0 and RZWQM2-DSSAT4.0 provided similar crop growth although the soil water balance simulations were different. Simulated yield- and biomass based-WUE were consistently lower in RZWQM2-DSSAT4.0 than in DSSAT4.0 due to higher ET simulated in RZWQM2-DSSAT4.0. The RZWQM2-DSSAT4.0 was further evaluated for the effects of estimated saturated hydraulic conductivity (Ksat) and Brooks-Corey soil water retention curve (SWRC) on crop growth. The two methods of estimating Ksat in RZWQM2-DSSAT4.0 were soil texture based average Ksat and effective porosity based Ksat. The two methods of obtaining SWRC were based on known soil water contents at 33 kPa and 1500 kPa pressure heads, and soil water content at 33 kPa only. For the six crops tested, simulation results showed that soil water and nitrogen (N) balances were more affected by Ksat than by SWRC, whereas simulated crop growth and WUE were more affected by SWRC than by Ksat. An evaluation of the RZWQM2-DSSAT4.0 hybrid model showed that the soil root growth factor (SRGF) can be estimated from an exponential equation with maximum rooting depth of 200 cm and an exponent of 3.0 in the literature, which removing the need to arbitrarily calibrate the SRGF for each soil layer. Crop WUE simulated with this new SRGF factor for all six crops was not significantly different from the original SRGF factors, but simulated aboveground biomass was improved with the new SRGF factors across the six crops. These simulation results should benefit RZWQM2-DSSAT4.0 users on how to use and parameterize the hybrid model for systems research and to understand crop growth and WUE from different models and model calibrations.