Soil Water Content Simulations Using a Simple Balance Model in the Wet Pampas

A simple balance model was used to simulate soil water content during the development of maize cops (Zea mays L.) in the area of Balcarce, Buenos Aires province (Argentina). Data were collected during a field experiment at the Unidad Integrada Facultad de Ciencias Agrarias UNMdP-EEA INTA, during the 1998-99 growing season. Maize plot was split into two treatments. In one of them the soil was covered in order to prevent soil evaporation (RRRC). In the other one the soil remains uncovered (RRRD). Both treatments were irrigated by sprinkling method to maintain an available water level for potential growing. During the simulated period the amount of rainfall was of 93.1mm and the plots were irrigated with 225.3 mm (RRRD) and 320.0 mm (RRRC) of water. Soil water content was measured at 2-5 days interval by the gravimetric method in the 0-0.1 m layer and by the neutron scattering method in the layers of 0.1-1.40 m at RRRC and 0.1-1.00 m at RRRD. Meteorological data were collected at the INTA Balcarce agro meteorological station, sited 300 m away from the plots. Also, aerial biomass of plants was samplet six times during growing season on some special phenological stages. Total green leaf area of the sample was estimated from green leaf area of a subsample measured with an area meter. Field studies were conducted on a Balcarce loam (illitic thermic loam petrocalcic Paleudoll) and 130 m above sea level. Balcarce has an annual average precipitation of 910 mm. Simulation was performed from 11/27/1998 to 03/01/1999. The seedtime was 10/16/1998. The proposed balance equation is: Cw(i)=(1/ra(i))*[Cw(i-1)ra(i-1)+P(i)+Ir(i)+Ära(i)Cwo(i-1)-Pe(i)-EET(i)-Es(i)] where i is a daily time index, Cw is the soil water content in the root zone, ra is the active root depth, Cwo is the soil water content in the zone without root, P is the rainfall, Ir is the irrigation, Ära is the variation in the root depth, Pe is percolation, EET is the real evapotranspiration (RRRD) or crop transpiration (RRRC), and Es is the surface runoff. Rainfall and irrigation were measured variables. Daily root growth depth was represented by a sinusoidal time function, with a maximum depth value reached at the bloom stage. Percolation was estimated as an instantaneous drainage to the passive root depth of the exceeded water whenever the water content reached field capacity. Daily values of surface runoff were estimated using the curve number methodology proposed by the USDA - Soil Conservation Service- National Engineering Handbook (1973). It was considered row crops land use conditions, with straight row practice and good hydrologic conditions. A type B hydrologic soil group was considered. Evapotranspiration and crop transpiration was determined using a simpler two-layer model, which calculate separately the evaporation (E) and transpiration (T). The model is based on a ‘parallel arrangement' of resistances, between the heat fluxes from the soil surface and the heat fluxes from the leaves in the canopy. The fraction of total available water that a crop can extract from the root zone necessary to obtain potential growing conditions was considered above 65% during the period of December 15th to January 31, and above 50 % in the rest of days. Simulation results were compared with the observed values for the total soil water content of the soil depth studied. The model represents adequately daily variations of soil water content at both plots. The values for determinant coefficient and mean square error are 0.930 and 0.0018 for RRRD and 0.965 and 0.0009 for RRRC of soil water content simulations.