EVAPORATION AND DISCHARGE FROM ARABLE LAND WITH CROPPED OR BARE SOILS DURING WINTER - MEASUREMENTS AND SIMULATIONS

Evaporation from cropping systems with and without crop cover during winter (spring cereals with and without undersown Italian rye grass) was studied. Measured daily discharge from field lysimeters and tile-drained plots was used to investigate to what extent discharge measurements could reveal differences in evaporation between the cropping systems. The identified differences were interpreted using a physically based model for simulating water and heat flows in the soil. Measurements were made between 1 April 1988 and 1 April 1991 on a sandy loam in southwestern Sweden. Annual precipitation (1 April-31 March) was 910 mm, 697 mm and 677 mm respectively. Total annual discharge showed a large degree of inconsistent variation between field plots within the same year and treatment, whereas the discharge dynamics showed more consistent differences between treatments. Standard meteorological variables and data on soil properties and crop development were used as input to the model. Calculations of soil evaporation, transpiration and evaporation of intercepted water were based on the Penman-Monteith equation. The model was calibrated against the actual date on which discharge started, which consistently occurred later in the autumn in the cropped soil than in the bare soil. Simulated annual total evaporation amounted to 505 mm, 470 mm and 396 mm from the system with bare soil during winter, and 552 mm, 510 mm and 423 mm from the system with cropped soil during winter. However, during periods with low temperatures and frequent precipitation, total evaporation rates for the bare soil were as high as or even exceeded those for cropped soil. Simulated annual soil evaporation from the system with cropped soil during winter constituted about 38% of the total annual evaporation, whereas it varied between 55 and 65% for the system with bare soil during winter. Discrepancies between simulated and measured discharge could have been due to spatial variability in soil and plant properties in the field, as well as to errors in the estimated unsaturated hydraulic conductivities or root water uptake functions used in the simulations. In simulations based on either modified soil properties or root depths, similar changes in water dynamics were obtained, demonstrating the need for independent determinations of the soil hydraulic properties.