Effect of meteorological conditions and water management on hydrological processes in agricultural fields : parameterization and modeling of Estonian case studies
Year: 2002
Type: dissertation
Abstract: This thesis was a study of the effect of meteorological conditions and water management on hydrological processes in agricultural fields. The emphasis was put on parameterization, validation and critical analysis of the different methods commonly used in field-scale hydrological models.\n\nThe study object was a two dimensional soil profile covered with cultivated grass that completely shaded the ground. Both energy and water balance components were estimated and compared with observed values and experimental results. A standard procedure for estimating net radiation was parameterized with new values to get a better fit with observed radiation in Estonia. The commonly used set of parameters were found to systematically overestimate net radiation during the summer months and underestimate in winter months. New equations for the net long-wave radiation that may also be used in the Priestley-Taylor equation were developed.\n\nSoil heat flux was estimated numerically for both bare soil and grass covered soil. The soil heat flux from grass-covered surfaces was less than 10% of the net radiation during the March-September period. The highest soil heat flux, in June (21 MJ m-2 month-1), was equal to 5.8% of net radiation. The largest relative value, in October (-13.7 MJ m-2 month-1), was equal to -173.4% of net radiation.\n\nMeasured evapotranspiration obtained from the hydraulic pan covered with a clipped grass canopy was used to validate the Penman-Monteith equation in Estonian conditions. In three out of four years the results were very good. The highest coefficient of determination was obtained in May 1985 (r2=0.922), and the lowest in July 1988 (r2=0.421). On a monthly basis comprising all years of experimentation the correlation was the best in June (r2=0.913). It was also shown that vapor pressure deficit correlated well with net radiation both on a daily basis (r2=0.602) and on a long-term monthly basis (r2=0.976). These results validate the use of the Priestly-Taylor equation in Estonian conditions. A comparison of the measured and estimated evapotranspiration revealed a higher r2 with the Penman-Monteith method. However the difference between the two methods was small to negligible in several months.\n\nWater retention curves and soil hydraulic conductivity functions were determined with two methods: 1) Wind's evaporation method and 2) Andersson's method based on the soil particle size distribution. Wind's method yielded a rather smooth curve for Θ(h) and a scattered cloud for K(h). The shape of the water retention curves were unexpectedly of a 'clay'-type, although the clay fraction was small in the samples. Andersson's method resulted in 'loamy soil'-type curves.\n\nThe experiment of controlled drainage by raising the water table at a nearby ditch showed that with very simple and low-cost hydraulic structures the water regime in an adjacent field could be affected. The agro-hydrological model, SWAP, developed in the Netherlands, was used to simulate the controlled drainage experiment. Calculated depths of the groundwater table and drainage flux using SWAP and CROPWATN (developed in Finland) were almost identical. The effect of different drainage design parameters, soil properties and different water management strategies was revealed using CROPWATN for continuous simulations over a period of 30 years.
Source: Aaltodoc (Aalto University)
Author: Toomas Tamm
Language: English
Topic: Soil and Unsaturated Flow
Subfield: Civil and Structural Engineering
Field: Engineering
Domain: Physical Sciences
SDGs: Climate action, Zero hunger
Open Access status: green
Funders: Helsingin Yliopisto, Eesti Teadusfondi
OpenAlex ID: W2596830355