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Title: Measurement and modeling of soil-water dynamics and evapotranspiration of drained peatland soils

Author
item SCHWARZEL, KAI - DRESDAN UNIV OF TECH
item SIMUNEK, JIRKA - UC, RIVERSIDE
item Van Genuchten, Martinus
item WESSOLEK, GERD - BELIN UNIV OF TECH

Submitted to: Journal of Plant Nutrition and Soil Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/23/2006
Publication Date: 12/8/2006
Citation: Schwarzel, K., Simunek, J., Van Genuchten, M.T., Wessolek, G. 2006. Measurement and modeling of soil-water dynamics and evapotranspiration of drained peatland soils. Journal of Plant Nutrition and Soil Science. Vol. 169(6):762-774

Interpretive Summary: Understanding the processes that control the retention and movement of water in peat soils is critical to effective management of such soils from both agricultural and ecological perspectives. Peat soils can serve as important sinks for water, nutrients and carbon, but also can pose major problems to the environment when they are drained. To successfully manage and conserve peat soils, it is important to understand their hydraulic behavior. In contrast to mineral soils, much less is known about peat soils, especially their hydraulic properties (including permeability), most likely due to the unique nature of the physical and hydraulic properties of peat soils, including volume changes upon dewatering. To better understand soil-water-plant relationships of peat soils, we conducted a series of lysimeter experiments in Northeastern Germany with peat soils naturally vegetated with mostly reed canary grass, and subjected to different groundwater levels. The data were analyzed in terms of a computer model that accounted for infiltration and drainage, upward flow from the water table, water uptake and evaporation. The numerical simulations closely followed the observed soil-moisture dynamics in the lysimeter and were consistent with measured evapotranspiration rates (water uptake plus evaporation from the soil surface). During dry years, evapotranspiration was found to depend mostly on the depth of the groundwater table. During wet years, evapotranspiration was found to be controlled more by the weather conditions, and not so much by the hydraulic properties of the peat soil and the depth to groundwater. Results of this study should be of interest to researchers, extension personnel, groundwater professionals, and others dealing with the management of peat soils.

Technical Abstract: Natural peat soils serve as important sinks for nutrients, organic components, and water. Peat soils can pose major environmental problems when they are drained for agricultural production, which may change their role in the landscape from a sink to a source. To successfully restore and conserve peat soils, it is important to understand the soilmoisture dynamics and water demand of drained peat soils for different climate and groundwater conditions. For this purpose, we conducted a series of lysimeter experiments with peat soils subject to different groundwater levels. Evapotranspiration (ET) rates and upward capillary fluxes in peat soils under grass were measured, while TDR probes and tensiometers were used to monitor the soil-water dynamics in the lysimeter during the growing season. The lysimeter data were simulated using an extended version of HYDRUS-1D to enable ET calculations using the Penman-Monteith equation. A physically based approach was tested to predict the canopy resistance as a function of the average pressure head of the soil root zone. The numerical simulations closely followed the observed soil-moisture dynamics in the lysimeter and were consistent with measured differences in ET rates for different groundwater levels. Besides average climate conditions, the effects of extreme dry and wet weather conditions on ET and groundwater recharge during the growing season were evaluated using the calibrated numerical model for different groundwater levels. Evapotranspiration rates during dry years depended very much on upward capillary flow from the water table and hence on the soil hydraulic properties. During wet years, however, ET was controlled mostly by the evaporative demand of the atmosphere, and much less by the soil hydraulic properties.