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31.
In sparsely cropped farming systems in semi-arid tropics, rainfall partitioning can be complex due to various interactions between vertical and horizontal water flows, both in the atmosphere and in the soil. Despite this, quantifying the seasonal rainfall partitioning is essential, in order to identify options for increased yields. Results are presented on water flow components, based on field measurements and water balance modelling, for three years (1994–96) in a farmer's field cultivated with pearl millet [Pennisetum glaucum (L.) Br.] in the Sahel (Niger). Water balance modelling was carried out for three common infiltration categories: runoff producing surfaces, surfaces receiving inflow of runon water from upstream zones, and a reference surface with zero runoff and runon. Runoff was calculated to 25%–30% of annual rainfall (which ranged from 488 to 596 mm), from crust observations, rainfall, soil wetness data, and infiltration estimates. Inflow of runon was estimated from field observations to 8%–18% of annual rainfall. The parameters in the functions for soil surface and canopy resistances were calibrated with field measurements of soil evaporation, stomatal conductance and leaf area. The model estimates of soil water contents, which were validated against neutron probe measurements, showed a reasonable agreement with observed data, with a root mean square error (RMSE) of approximately 0.02 m3 m−3 for 0–160 cm soil depth. Estimated productive water flow as plant transpiration was low, amounting to 4%–9% of the available water for the non-fertilised crop and 7%–24% for the fertilised crop. Soil evaporation accounted for 31%–50% of the available water, and showed a low variation for the observed range of leaf area (LAI <1 m2 m−2). Deep percolation was high, amounting to 200–330 mm for the non-crusted surfaces, which exceeded soil evaporation losses, for 1994–95 with relatively high annual rainfall (517–596 mm). Even a year with lower rainfall (488 mm) and a distinct dry spell during flowering (1996), resulted in an estimated deep percolation of 160 mm for the non-fertilised crop. The crop did not benefit from the additional inflow of runon water, which was partitioned between soil water storage and deep percolation. The only exception to this was the fertilised crop in 1996, where runon somewhat compensated for the limited rainfall and the higher water demand as a result of a larger leaf area than the non-fertilised crop. The effects of rainfall erraticness, resulting in episodic droughts, explain why a crop that uses such a small proportion of the available water, in an environment with substantial deep percolation, still suffers from water scarcity. Application of small levels of phosphorus and nitrogen roughly doubled yields, from 380 to 620 kg ha−1, and plant transpiration, from 33 to 78 mm. Evapotranspirational water use efficiency (WUEET) was low, 6500–8300 m3 ton−1 grain for non-fertilised crop, which is an effect of the low on-farm yields and high non-productive water losses. The estimated seasonal rainfall partitioning indicates the possibility of quantifying vertical water flows in on-farm environments in the Sahel, despite the presence of surface overland flow. 相似文献
32.
Understanding photosynthesis and plant water management as a coupled process remains an open scientific problem. Current eco-hydrologic models characteristically describe plant photosynthetic and hydraulic processes through ad hoc empirical parameterizations with no explicit accounting for the main pathways over which carbon and water uptake interact. Here, a soil–plant-atmosphere continuum model is proposed that mechanistically couples photosynthesis and transpiration rates, including the main leaf physiological controls exerted by stomata. The proposed approach links the soil-to-leaf hydraulic transport to stomatal regulation, and closes the coupled photosynthesis–transpiration problem by maximizing leaf carbon gain subject to a water loss constraint. The approach is evaluated against field data from a grass site and is shown to reproduce the main features of soil moisture dynamics and hydraulic redistribution. In particular, it is shown that the differential soil drying produced by diurnal root water uptake drives a significant upward redistribution of moisture both through a conventional Darcian flow and through the root system, consistent with observations. In a numerical soil drying experiment, it is demonstrated that more than 50% of diurnal transpiration is supplied by nocturnal upward water redistribution, and some 12% is provided directly through root hydraulic redistribution. For a prescribed leaf area density, the model is then used to diagnose how elevated atmospheric CO2 concentration and increased air temperature jointly impact soil moisture, transpiration, photosynthesis, and whole-plant water use efficiency, along with compensatory mechanisms such as hydraulic lift using several canonical forms of root-density distribution. 相似文献
33.
Large land areas in Sweden are planned to be planted with high producing, short rotation forest stands of willow in the beginning of the 1990s. Since willow is a highly hydrophilic species, this new land use may have strong implications on water resources. To assess these implications, evaporation of Salix viminalis and Salix viminalis x caprea stands in lysimeters was analysed with the simple, yet physically realistic KAUSHA model. Parameter values for the Lohammar equation were deduced (b = 100 m3 kg?1, kmax = 0.01 m s?1), believed to be applicable to other sites. Simulated evaporation during the 1980 growth season for a normal stand with a production of 12 tonnes of dry matter per hectare per season was 526 mm, of which 375 mm was transpiration, 56 mm interception evaporation, and 95 mm soil evaporation. For an optimally irrigated 20-tonnes stand, the total evaporation was 584 mm, of which 430 mm was transpiration. As a comparison, Penman open water evaporation was 430 mm. To avoid soil water stress in the 20-tonnes stand, 140 mm was needed as irrigation, equivalent to 25 per cent of the mean annual precipitation. Since intensively cultivated willow plantations seemed to be using much water, it was concluded that introduction of this agri-forestry practice must be carefully planned to make use of this property, e.g. in biological filters or in reclaiming water-logged land. 相似文献