Sugarcane transpiration with shallow water-table: sap flow measurements and modelling

Abstract

The most common sugarcane variety in the Gharb plain of Morocco (CP 66-345 variety) was grown in a lysimeter in the laboratory. It developed during 6 months with a water-table at 0.7 m below the soil surface. The water-table was then successively maintained with a Mariotte bottle at 0.45, 0.2 and 0.05 m from the soil surface for 21, 31 and 24 days, respectively. Transpiration was measured by Dynamax sap flow sensors. Soil water pressure heads were measured at six different depths; soil hydraulic properties and root density profile were also determined. No transpiration reduction was observed with soil waterlogging. Two different models were used to predict the pattern of root water uptake (RWU) with water-table at 0.45 m below the soil surface. These two models are based on a RWU function used as sink term in the Richards equation. The first model, HYDRUS-2D ( Simunek et al., 1996), is based on the α-model RWU ( Feddes et al., 1978a) which depends on a reduction function varying according to the soil water pressure head and on the root density. The second model, SIC ( Breitkopf and Touzot, 1992) is based on the h r-model RWU ( Whisler and Millington, 1968; Feddes et al., 1974). It is proportional to the difference between soil and root pressure heads, to unsaturated hydraulic conductivity and to root density. Calculated soil water flows from pressure head measurements are compared to predicted pressure heads by the two models. These predictions compare well with the measured values and show that sugarcane roots mainly absorbed water in the water-table. However, while goods predictions were obtained using the actual root density profile with the h r-model, it was necessary to modify this profile to obtain proper results using the α-model.