Ground cover influence on evaporation and stable water isotopes in soil water (original) (raw)
Related papers
Evaporation components of a boreal forest: variations during the growing season
Journal of Hydrology, 1997
To improve the understanding of interactions between the boreal forest and the climate system as a key issue for global climate change, the water budget of a mixed pine and spruce forest in central Sweden was estimated by measurements of the water flux components and the total evaporation flux during the period 16 May-31 October 1995. Total evaporation was measured using eddy correlation and the components were obtained using measurements of precipitation, throughfall, tree transpiration, and forest floor evaporation. On a daily basis, tree transpiration was the dominant evaporation component during the vegetation period. However, it could be efficiently blocked by a wet canopy associated with large interception evaporation. The accumulated total evaporation was 399 mm, transpiration was 243 mm, forest floor evaporation was 56 mm and interception evaporation was 74 mm. The accumulated sum of interception, transpiration, and floor evaporation was 51 mm larger than the actual measured total evaporation. This difference was mainly attributed to the fact that transpiration was measured in a rather dense 50-year-old stand while total evaporation represented the average conditions of older, roughly 100-year-old stands. To compare eddy-correlation measurements with small-scale measurements of evaporation components, a source area analysis was made to select the flux data that give the best representation of the investigated stand. Especially under stable atmospheric conditions the requirements for surface homogeneity were very high and extreme care had to be taken to be aware of the flux source areas. Canopy water storage was determined by two methods: by the water balance of the canopy, which gave a result of 3.3 mm; and by the so-called minimum method based on plots of throughfall versus precipitation, which gave a much lower value of 1.5 mm. Seasonal interception evaporation constituted 30% of the precipitation.
The degree of soil water saturation in the narrow-leaved ash (Fraxinus angustifolia Vahl)
Ekologia Bratislava, 2007
Research is aimed at analysing the dynamics of the soil water saturation degree (FDR method) during three characteristic developmental phases of ash stands. The studied stands are located in the lowland forest in Upper Posavina (a part of the Sava river valley) in Croatia. The experiment was established in natural stands of a community of narrow-leaved ash with autumn snowflake (Leucoio-Fraxinetum angustifoliae G l a v. 1959), which is distributed in the area dominated by the community of pedunculate oak and great greenweed (Genisto elatae-Quercetum roboris R a u š 1969). Three locations in a micro-depression of a flooded plain were chosen: 1) the lowest microrelief position-the bottom of a bogged site-initial development phase; 2) the central micro-relief position-optimal development phase; 3) the marginal bogged area towards a fresh micro-depression-terminal development phase. The degree of water content in the soil is regarded a very practical indicator of ash stand conditions, in which the soil in the lower part of the rhizosphere (in the initial phase it is within a depth of 1 m) is completely saturated with water over most of the year or the whole year. Anaerobiosis, or reduction conditions in the soil correspond to the soil saturation degree. In terms of the degree of soil water saturation, the terminal phase of ash forest is the most distinct, while in terms of total water quantity in the profile and its dynamics, the initial phase takes up the first place. Based on research we can concluse that developmental phases differ from one another in the dynamics of soil water saturation at almost all depths. At the beginning of a drier vegetation period there was no difference between the initial and the optimal phase in the bare regeneration area, and between the terminal and the optimal phase in the old stand. Similarity between these pairs was evident throughout the vegetation period. In the year with a more humid vegetation period, this differentiation regularity was hardly evident in terms of saturation degree dynamics. In conditions of a drier vegetation period (2000), the most favourable distribution of water in the soil occurred in the terminal phase.
Soil water dynamics and long-term water balances of a Douglas fir stand in the Netherlands
Journal of Hydrology, 1994
Seasonal dynamics of soil water, the annual water balances over a period of 30 years (1960 1990) and the frequency of occurrence of extreme water shortage were quantified with the model 'SWIF' for a Douglas fir stand on a sandy soil in the Netherlands. This information was needed for evaluating the impact of air pollution and water shortage on forest growth, and to calculate chemical budgets. Measurements necessary for model parameterization included rainfall, throughfall, transpiration, soil water content and soil water pressure head. These data are available for 3 years. For the period without measurements, throughfall and potential evapotranspiration were calculated from synoptic weather data with an empirical model. The effect of using synoptic weather data instead of detailed on-site measured data was analysed. The results show that after calibration of the empirical model the simulated annual water balance and seasonal soil water dynamics were well described with synoptic weather data as well as with detailed on-site measured data. The modelled annual water balances for the period 1960-1990 are presented as cumulative frequency distributions. Median values of the major terms of the water balance and the frequency of occurrence of extreme water shortage were derived from these diagrams. Annual median precipitation was 834 ram, calculated median interception loss 317 ram, r alan transpiration 363 mm, median soil water evaporation 32 mm and median vertical drainage at 150 cm depth 195 mm. The sum of the major losses is not equal to median annual precipitation as the median values of the individual terms may occur in different years. Transpiration shows smaller variations (11%) between the years than throughfall (54%) and vertical drainage (112%). Although the median value of transpiration reduction resulting from water stress is low (4%), some years showed extreme water shortage, the highest transpiration reduction being 31% for 1976. Although total transpiration reduction during median years is low, short periods with considerable drought stress occur during these years.
Journal of Arid Environments, 2010
Soil evaporation, a critical ecohydrological process in drylands, can exhibit substantial spatio-temporal variation. Spatially, ecohydrological controls of soil evaporation may generally depend on a hierarchical structure spanning from the presence or absence of litter, through canopy patches of woody plants and intercanopy patches separating them, up to the overall vegetation mosaic characterized by density of woody plant cover in the landscape, although assessment of these factors in concert is generally lacking. Temporally, ecohydrological controls can be further complicated by not only seasonal climate, but also phenology, particularly in seasonally deciduous drylands. We experimentally assessed the interactive controls on soil evaporation along a gradient of mesquite cover (Prosopis velutina) within the North American monsoon region, with respect to such hierarchical structure and seasonality/phenology. Our results indicate that presence of litter exerts a dominant control on soil evaporation, independent of seasonality; in absence of litter, both patch and mosaic attributes influence soil evaporation variably with season/phenology. Correlations from related measures of incoming energy suggest energy limits evaporation in many cases, although other factors such as wind may potentially influence hierarchical and seasonal/phenological combinations. Our results highlight the need to account for both hierarchical vegetation structure and seasonal/phenological variability to improve ecohydrological predictions of soil evaporation.
Polish Journal of …, 2009
European beech (Fagus sylvatica L.) ranks as one of the most adaptive species among European indigenous trees. Variable interactions between the trees and soil water depend on both phenotypic plasticity of the species and natural conditions. They are controlled through stomatal regulation and the ability of beech trees to accelerate quickly their growth if available resources increase. However, the effect of forest density at various altitudes on the soil water content in beech stands has been studied rather scarcely. Therefore, we monitored soil moisture by means of Time Domain Reflectometry in series of natural and managed stands located on sites representing the lower altitude (200-550 m a.s.l.), middle altitude (550-1050 m a.s.l.) and higher altitude (1050-1300 m a.s.l.) zones of the natural beech belt in the Western Carpathians, Slovakia. Forest stand density, expressed in terms of basal area, i.e. the sum of cross section areas of the tree stems at 1.30 m height, was unchanged in natural stands, but it was reduced by 60% in the shelterwood stands. In the clear-cuts, all trees were removed. Total soil water content (SWC) under forest stands was calculated in mm as the product of soil moisture and soil depth, the latter acquired by electrical resistivity tomography. SWC differences between natural and shelterwood stands of the lower altitude, middle altitude and higher altitude zones averaged 18 mm, 36 mm and-3 mm, respectively. According to the Friedman test on ranks, followed by post-hoc multiple comparison testing, the difference was only significant within the middle altitude zone. In it, soil water consumption by the natural stand was limited only by the hormonally controlled seasonal regulation. The comparatively low water loss in the shelterwood stand resulted from a small rainfall interception by forest canopy and a decreased soil water uptake due to reduced basal area, leaf area index and simple age-size forest structure. In the lower altitude zone, the precipitation deficit and limited extractability of soil water were responsible for the absence of larger SWC differences. As opposed to that, low potential evapotranspiration prevented any noticeable SWC differences within the higher altitude zone.
Journal of Hydrology, 2004
Automated time domain reflectometry (TDR) measurements in high resolution over soil depth and over time were performed in a mixed beech-spruce and a spruce stand during two hydrologically contrasting seasons. Soil drying was more intensive and reached deeper soil layers in the mixed stand, which on the other hand allowed more stand precipitation, compensating for the higher evapotranspiration rates. These results were confirmed by a large number of spatially distributed TDR measurements along grids of different spacing, which additionally covered a beech stand. Spatial water depletion patterns of the topsoil in spring appeared to be largely congruent with tree species distribution and reflected the higher water consumption of fully foliated beech. Variability was highest in the mixed stand, where a spatial correlation within a range of about 7 m was observed. The pure stands lacked spatial correlation. The effect of the mixed stand on soil water depletion and recharge turned out to be non-additive as compared to the pure stands of beech and spruce: changes of soil water storage under the mixed stand almost equalled the values measured in the beech stand. During selected drying periods in 2000 average daily water extraction rates from the uppermost 60 cm of soil amounted to 1.65 mm in the beech as well as in the mixed stand, which is about 45% more than under pure spruce. Maximum differences of up to 84% occurred in periods with high evaporative demand. The overproportionate evapotranspiration of the mixed stand was exclusively attributable to beech, which deepened and intensified its fine-root system in mixture, while spruce rooted more shallowly. The mixed stand extracted a higher percentage of water from deeper soil layers than the pure stands. q
2002
In forest the soil water balance is strongly influenced by tree species composition. For example, differences in transpiration rate lead to differences in soil water storage (SWS) and differences in canopy interception cause differences in infiltration. To analyse the influence of tree species composition on SWS at the scale of a forest stand, we compare spatio-temporal patterns in vegetation and SWS. Geostatistical space-time models provide a probabilistic framework for mapping SWS from point observations. The accuracy of these models may be improved by incorporating knowledge about the process of evapotranspiration. In this paper we combine a physical-deterministic evapotranspiration model with space-time geostatistical interpolation to predict soil water storage in the upper 30 cm of soil (SWS 30) for a 0.5 ha plot in a mixed stand of Norway spruce (Picea abies (L.) Karst.) and European beech (Fagus sylvatica L.) in Kreisbach, Lower Austria. Soil water storage was measured at 198 locations by permanently installed wave guides. This was repeated 28 times, about every two weeks during the growing seasons of 2000 and 2001. Incorporation of a process-based model in space-time prediction of SWS 30 reduced the effect of precipitation on SWS 30 predictions prior to precipitation. Spatial patterns of SWS 30 between the permanent wilting point and field capacity depend on the precipitation and drying history, which is affected by vegetation. Early in the growing season spruce starts to transpire markedly, which is common for coniferous trees. During dry periods, spruce reduces transpiration earlier than beech. Overall beech transpires more than spruce during the growing season. The greater transpiration rates of beech are compensated for by greater soil water recharge after precipitation because less rainfall is intercepted. At low water contents near the permanent wilting point SWS 30 was spatially quite uniform. This was also the case at water contents nearfield capacity, probably because the soil physical parameters varied little. Space-time interpolation of SWS 30 and the prediction of soil water discharge and soil water recharge during periods of drying and rewetting demonstrate the important role of vegetation on the spatial patterns of SWS 30 .
Extinction Depth and Evapotranspiration from Ground Water under Selected Land Covers
Ground Water, 2007
In many landscapes, vegetation extracts water from both the unsaturated and the saturated zones. The partitioning of evapotranspiration (ET) into vadose zone evapotranspiration and ground water evapotranspiration (GWET) is complex because it depends on land cover and subsurface characteristics. Traditionally, the GWET fraction is assumed to decay with increasing depth to the water table (DTWT), attaining a value of 0 at what is termed the extinction depth. A simple assumption of linear decay with depth is often used but has never been rigorously examined using unsaturated-saturated flow simulations. Furthermore, it is not well understood how to relate extinction depths to characteristics of land cover and soil texture. In this work, variable saturation flow theory is used to simulate GWET for three land covers and a range of soil properties under drying soil conditions. For a water table within half a meter of the land surface, nearly all ET is extracted from ground water due to the close hydraulic connection between the unsaturated and the saturated zones. For deep-rooted vegetation, the decoupling of ground water and vadose zone was found to begin at water table depths between 30 and 100 cm, depending on the soil texture. The decline of ET with DTWT is better simulated by an exponential decay function than the commonly used linear decay. A comparison with field data is consistent with the findings of this study. Tables are provided to vary the extinction depth for heterogeneous landscapes with different vegetation cover and soil properties.