Daily Soil Water Research Articles

Performance evaluation of evapotranspiration estimations in a model of soil water balance

AbstractSoil water content models have huge applications from an agronomic point of view and they are usually used as a sub‐model for weather and climate modelling. They are also useful tools for efficient water management irrigation practices. The aim of this investigation is to evaluate the performance of two different parameterizations of evapotranspiration when applied to a soil water balance model. Experimental data of a maize crop is used to evaluate model accuracy. The first methodology proposes a parallel resistance arrangement to represent the latent heat fluxes of the soil surface and the leaves in the canopy layer considering the leaf area index (LAI). The second methodology uses the parameterization proposed by the United Nations Food and Agriculture Organization (FAO), based on the crop coefficient (Kc) and the potential evapotranspiration obtained from the Penman–Monteith equation. The crop was divided into five plots with different irrigation systems according to their phenological stages. The model suitably predicts daily soil water content in five different irrigation systems. Predictions of soil water content using the LAI or Kc methodology tend to overestimate observations. In addition, the model has better predictions using the LAI methodology than the Kc methodology. The root mean square error and the determination coefficient were 0.059 and 0.92, respectively, with the LAI methodology and 0.063 and 0.87, respectively, using the Kc methodology. Copyright © 2010 Royal Meteorological Society

Assessing satellite-based basal crop coefficients for irrigated grapes (Vitis vinifera L.)

Abstract A combined methodology of basal crop coefficient ( K cb ) derived from vegetation indices (VI) obtained from satellite images and a daily soil water balance in the root zone of the crop was proposed to accurately estimate the daily grape crop coefficient and actual evapotranspiration. The modeled values were compared with field measurements of crop evapotranspiration (ET) using an energy balance eddy-covariance flux tower and adjusted for closure using the measured Bowen ratio. A linear relation between K cb and VI for vineyard was obtained, K cb = 1.44 × NDVI-0.10 and K cb = 1.79 × SAVI-0.08. The correlation of the measured crop coefficient ( K c ) and modeled ( K crf ) exhibits a linear tendency, K c = 0.96 K crf , r 2 = 0.67. Other derived parameters such as weekly K c and daily and weekly ET show good consistency with measurements and higher coefficients of determination. The study of the soil water balance suggests the importance of soil water storage in grapes within the La Mancha region. These results validate the use of remote sensing as a tool for the estimation of evapotranspiration of irrigated wine grapes planted on trellis systems.

Irrigation scheduling performance by evapotranspiration-based controllers

Abstract Evapotranspiration-based irrigation controllers, also known as ET controllers, use ET information or estimation to schedule irrigation. Previous research has shown that ET controllers could reduce irrigation as much as 42% when compared to a time-based irrigation schedule. The objective of this study was to determine the capability of three brands of ET-based irrigation controllers to schedule irrigation compared to a theoretically derived soil water balance model based on the Irrigation Association Smart Water Application Technologies (SWAT) protocol to determine the effectiveness of irrigation scheduling. Five treatments were established, T1–T5, replicated four times for a total of twenty field plots in a completely randomized block design. The irrigation treatments were as follows: T1, Weathermatic SL1600 with SLW15 weather monitor; T2, Toro Intelli-sense; T3, ETwater Smart Controller 100; T4, a time-based treatment determined by local recommendations; and T5, a reduced time-based treatment 60% of T4. All treatments utilized rain sensors set at a 6 mm threshold. A daily soil water balance model was used to calculate the theoretical irrigation requirements for comparison with actual irrigation water applied. Calculated in 30-day running totals, irrigation adequacy and scheduling efficiency were used to quantify under- and over-irrigation, respectively. The study period, 25 May 2006 through 27 November 2007, was drier than the historical average with a total of 1326 mm of rainfall compared to 1979 mm for the same historical period. It was found that all treatments applied less irrigation than required for all seasons. Additionally, the ET controllers applied only half of the irrigation calculated for the theoretical requirement for each irrigation event, on average. Irrigation adequacy decreased when the ET controllers were allowed to irrigate any day of the week. All treatments had decreased scheduling efficiency averages in the rainy season with the largest decrease of 29 percentile points with a timer and rain sensor (T4) and an average decrease of 20 percentile points for the ET controllers, indicating that site specific rainfall has a significant effect on scheduling efficiency results. Rainfall did not drastically impact the average irrigation adequacy results. For this study, there were two controller program settings that impacted the results. The first setting was the crop coefficients where specific values were chosen for the location of the study when calculating the theoretical requirement whereas the controllers used default values. The second setting was the soil type that defines the soil water holding capacity of the soil. The ET controllers were able to regularly adjust to real-time weather, unlike the conventional irrigation timers. However, the incorporation of site specific rainfall measurements is extremely important to their success at managing landscape water needs and at a minimum a rain sensor should be used.

Actual evapotranspiration assessment by means of a coupled energy/hydrologic balance model: Validation over an olive grove by means of scintillometry and measurements of soil water contents

A coupled energy/hydrologic model was applied to simulate the exchange of energy and water in the soil–plant-atmosphere system (SPA). The model, which uses a “two-source” approach to estimate the energy fluxes, and the “force-restore” approach to represent the water balance, was validated by means of evapotranspiration measurements collected via scintillometry and soil moisture measurements collected via time domain reflectometry (TDR) in a Sicilian olive grove. The comparison between measured and estimated fluxes values at an hourly scale showed good agreement. Additional comparisons on a daily timescale confirmed the model’s applicability for quantifying crop water requirements. Also in terms of daily evapotranspiration and soil water content values, the obtained results confirmed the model’s applicability for those practical applications aiming to quantify the crop water requirement. Moreover, further studies should be conducted to test the feasibility of using this model for long term simulations over a broad range of conditions.

A global and high-resolution assessment of the green, blue and grey water footprint of wheat

Abstract. The aim of this study is to estimate the green, blue and grey water footprint of wheat in a spatially-explicit way, both from a production and consumption perspective. The assessment is global and improves upon earlier research by taking a high-resolution approach, estimating the water footprint of the crop at a 5 by 5 arc minute grid. We have used a grid-based dynamic water balance model to calculate crop water use over time, with a time step of one day. The model takes into account the daily soil water balance and climatic conditions for each grid cell. In addition, the water pollution associated with the use of nitrogen fertilizer in wheat production is estimated for each grid cell. We have used the water footprint and virtual water flow assessment framework as in the guideline of the Water Footprint Network. The global wheat production in the period 1996–2005 required about 108 billion cubic meters of water per year. The major portion of this water (70%) comes from green water, about 19% comes from blue water, and the remaining 11% is grey water. The global average water footprint of wheat per ton of crop was 1830 m3/ton. About 18% of the water footprint related to the production of wheat is meant not for domestic consumption but for export. About 55% of the virtual water export comes from the USA, Canada and Australia alone. For the period 1996–2005, the global average water saving from international trade in wheat products was 65 Gm3/yr. A relatively large total blue water footprint as a result of wheat production is observed in the Ganges and Indus river basins, which are known for their water stress problems. The two basins alone account for about 47% of the blue water footprint related to global wheat production. About 93% of the water footprint of wheat consumption in Japan lies in other countries, particularly the USA, Australia and Canada. In Italy, with an average wheat consumption of 150 kg/yr per person, more than two times the word average, about 44% of the total water footprint related to this wheat consumption lies outside Italy. The major part of this external water footprint of Italy lies in France and the USA.

Diffuse and concentrated recharge evaluation using physical and tracer techniques: results from a semiarid carbonate massif aquifer in southeastern Spain

In the high-permeability, semiarid carbonate aquifer in the Sierra de Gador Mountains (southeastern Spain), some local springs draining shallow perched aquifers were of assistance in assessing applicability of the atmospheric chloride mass balance (CMB) for quantifying total yearly recharge (RT) by rainfall. Two contrasting hydrological years (October through September) were selected to evaluate the influence of climate on recharge: the average rainfall year 2003–2004, and the unusually dry 2004–2005. Results at small catchment scale were calibrated with estimated daily stand-scale RT obtained by means of a soil water balance (SWB) of rainfall, using the actual evapotranspiration measured by the eddy covariance (EC) technique. RT ranged from 0.35 to 0.40 of rainfall in the year, with less than a 5% difference between the CMB and SWB methods in 2003–2004. RT varied from less than 0.05 of rainfall at mid-elevation to 0.20 at high elevation in 2004–2005, with a similar difference between the methods. Diffuse recharge (RD) by rainfall was quantified from daily soil water content field data to split RT into RD and the expected concentrated recharge (RC) at catchment scale in both hydrological years. RD was 0.16 of rainfall in 2003–2004 and 0.01 in 2004–2005. Under common 1- to 3-day rainfall events, the hydraulic effect of RD is delayed from 1 day to 1 week, while RC is not delayed. This study shows that the CMB method is a suitable tool for yearly values complementing and extending the more widely used SWB in ungauged mountain carbonate aquifers with negligible runoff. The slight difference between RT rates at small catchment and stand scales enables results to be validated and provides new estimates to parameterize RT with rainfall depth after checking the weight of diffuse and concentrated mechanisms on RT during moderate rainfall periods and episodes of marked climatic aridity.

Ecophysiological responses of black walnut (Juglans nigra) to plantation thinning along a vertical canopy gradient

Abstract Ecophysiology of black walnut ( Juglans nigra L.) along a vertical canopy gradient was studied in a mixed species plantation for the first (2007) and second (2008) growing seasons after thinning to understand response mechanisms to increased resource availability. Parameters were affected by the thinning treatment in 2008 only. Thinned trees showed increased light-saturated maximum photosynthesis ( A max ) from 2007 to 2008 compared to non-thinned trees. This response was likely due to increased light levels near thinned trees (vs. changes in water or nutrient status), because thinning did not affect midday leaf water potential ( Ψ md ), average daily soil water content (SWC), or leaf nitrogen content per unit area ( N a ). Plantation thinning did not increase relative diameter growth during the experimental period. This may be due to low thinning intensity and mortality prior to thinning that reduced competition from first-tier neighbors. Certain leaf traits such as leaf mass per unit area (LMA) and N a increased from the bottom to the upper canopy position, but did not influence thinning responses. Distribution patterns of photosynthetic parameters through the vertical canopy gradient were less defined than leaf structural traits such as LMA and N a . Findings reflect black walnut's large variability in response to thinning.

Technical Note: Scheduling for Deficit Irrigation—Crop Yield Predictor

Irrigators in many countries with dwindling water supplies face the prospect that they will not be able to fully irrigate their crops. In these cases, they still need to schedule their water applications to make the best economic use of available water. Major scheduling questions for deficit irrigation include: (1) will pre-season irrigation be beneficial; and (2) when should irrigation be started and stopped during the growing season. Traditional irrigation scheduling estimates crop evapotranspiration (ETc) to predict the amount and timing of irrigation events for the next several days. Usually these schedules assume that the water supply will provide irrigation to fully irrigate the crop to produce maximum crop yields. Irrigators practicing deficit irrigation need to predict irrigation schedules in advance of the growing season and make appropriate adjustments based on potential crop yields and economic returns. A computerized decision tool, the Crop Yield Predictor (CYP), has been developed to forecast yields from alternative irrigation schedules and designed for management decisions by irrigators, crop consultants, and extension personnel. Users of CYP determine soil water status before or during the cropping season and formulate potential schedules of irrigation dates and amounts. Soil water holding capacity and irrigation system water delivery capacity are constraints on the ability to supply water to the crop. CYP uses a daily soil water balance coupled with computations of effective evapotranspiration (ETe) to predict crop yields from regional yield-ET relationships. Multiple executions of CYP with alternative irrigation schedules lead to the schedules that project optimum net economic returns from the management scenarios. CYP is an example of adapting a crop simulation model into a tool for those who need to make irrigation management decisions.

The spatial and temporal variability of groundwater recharge in a forested basin in northern Wisconsin

AbstractRecharge varies spatially and temporally as it depends on a wide variety of factors (e.g. vegetation, precipitation, climate, topography, geology, and soil type), making it one of the most difficult, complex, and uncertain hydrologic parameters to quantify. Despite its inherent variability, groundwater modellers, planners, and policy makers often ignore recharge variability and assume a single average recharge value for an entire watershed. Relatively few attempts have been made to quantify or incorporate spatial and temporal recharge variability into water resource planning or groundwater modelling efforts. In this study, a simple, daily soil–water balance model was developed and used to estimate the spatial and temporal distribution of groundwater recharge of the Trout Lake basin of northern Wisconsin for 1996–2000 as a means to quantify recharge variability. For the 5 years of study, annual recharge varied spatially by as much as 18 cm across the basin; vegetation was the predominant control on this variability. Recharge also varied temporally with a threefold annual difference over the 5‐year period. Intra‐annually, recharge was limited to a few isolated events each year and exhibited a distinct seasonal pattern. The results suggest that ignoring recharge variability may not only be inappropriate, but also, depending on the application, may invalidate model results and predictions for regional and local water budget calculations, water resource management, nutrient cycling, and contaminant transport studies. Recharge is spatially and temporally variable, and should be modelled as such. Copyright © 2009 John Wiley & Sons, Ltd.

A method for spatial prediction of daily soil water status for precise irrigation scheduling

Available water holding capacity (AWC) and field capacity (FC) maps have been produced using regression models of high resolution apparent electrical conductivity (EC a) data against AWC (adj. R 2 = 0.76) and FC (adj. R 2 = 0.77). A daily time step has been added to field capacity maps to spatially predict soil water status on any day using data obtained from a wireless soil moisture sensing network which transmitted hourly logged data from embedded time domain transmission (TDT) sensors in EC a-defined management zones. In addition, regular time domain reflectometry (TDR) monitoring of 50 positions in the study area was used to assess spatial variability within each zone and overall temporal stability of soil moisture patterns. Spatial variability of soil moisture within each zone at any one time was significant (coefficient of variation [% CV] of volumetric soil moisture content ( θ) = 3–16%), while temporal stability of this pattern was moderate to strong (bivariate correlation, R = 0.52–0.95), suggesting an intrinsic soil and topographic control. Therefore, predictive ability of this method for spatial characterisation of soil water status, at this site, was limited by the ability of the sensor network to account for the spatial variability of the soil moisture pattern within each zone. Significant variability of soil moisture within each EC a-defined zone is thought to be due to the variable nature of the young alluvial soils at this site, as well as micro-topographic effects on water movement, such as low-lying ponding areas. In summary, this paper develops a method for predicting daily soil water status in EC a-defined zones; digital information available for uploading to a software-controlled automated variable rate irrigation system with the aim of improved water use efficiency. Accuracy of prediction is determined by the extent to which spatial variability is predicted within as well as between EC a-defined zones.

Quantifying blue and green virtual water contents in global crop production as well as potential production losses without irrigation

Crop production requires large amounts of green and blue water. We developed the new global crop water model GCWM to compute consumptive water use (evapotranspiration) and virtual water content (evapotranspiration per harvested biomass) of crops at a spatial resolution of 5′ by 5′, distinguishing 26 crop classes, and blue versus green water. GCWM is based on the global land use data set MIRCA2000 that provides monthly growing areas for 26 crop classes under rainfed and irrigated conditions for the period 1998–2002 and represents multi-cropping. By computing daily soil water balances, GCWM determines evapotranspiration of blue and green water for each crop and grid cell. Cell-specific crop production under both rainfed and irrigated conditions is computed by downscaling average crop yields reported for 402 national and sub-national statistical units, relating rainfed and irrigated crop yields reported in census statistics to simulated ratios of actual to potential crop evapotranspiration for rainfed crops. By restricting water use of irrigated crops to green water only, the potential production loss without any irrigation was computed. For the period 1998–2002, the global value of total crop water use was 6685 km 3 yr −1 , of which blue water use was 1180 km 3 yr −1 , green water use of irrigated crops was 919 km 3 yr −1 and green water use of rainfed crops was 4586 km 3 yr −1 . Total crop water use was largest for rice (941 km 3 yr −1 ), wheat (858 km 3 yr −1 ) and maize (722 km 3 yr −1 ). The largest amounts of blue water were used for rice (307 km 3 yr −1 ) and wheat (208 km 3 yr −1 ). Blue water use as percentage of total crop water use was highest for date palms (85%), cotton (39%), citrus fruits (33%), rice (33%) and sugar beets (32%), while for cassava, oil palm and cocoa, almost no blue water was used. Average crop yield of irrigated cereals was 442 Mg km −2 while average yield of rainfed cereals was only 266 Mg km −2 . Average virtual water content of cereal crops was 1109 m 3 Mg −1 of green water and 291 m 3 Mg −1 of blue water, while average crop water productivity of cereal crops was 714 g m −3 . If currently irrigated crops were not irrigated, global production of dates, rice, cotton, citrus and sugar cane would decrease by 60%, 39%, 38%, 32% and 31%, respectively. Forty-three per cent of cereal production was on irrigated land, and without irrigation, cereal production on irrigated land would decrease by 47%, corresponding to a 20% loss of total cereal production. The largest cereal production losses would occur in Northern Africa (66%) and Southern Asia (45%) while losses would be very low for Northern Europe (0.001%), Western Europe (1.2%), Eastern Europe (1.5%) and Middle Africa (1.6%). Uncertainties and limitations are discussed in the manuscript, and a comparison of GCWM results to statistics or results of other studies shows good agreement at the regional scale, but larger differences for specific countries.

Soil water status mapping and two variable-rate irrigation scenarios

Irrigation is the major user of allocated global freshwaters, and scarcity of freshwater threatens to limit global food supply and ecosystem function—hence the need for decision tools to optimize use of irrigation water. This research shows that variable alluvial soil ideally requires variable placement of water to make the best use of irrigation water during crop growth. Further savings can be made by withholding irrigation during certain growth stages. The spatial variation of soil water supplied to (1) pasture and (2) a maize crop was modelled and mapped by relating high resolution apparent electrical conductivity maps to soil available water holding capacity (AWC) at two contrasting field sites. One field site, a 156-ha pastoral farm, has soil with wide ranging AWCs (116–230 mm m−1); the second field site, a 53-ha maize field, has soil with similar AWCs (161–164 mm m−1). The derived AWC maps were adjusted on a daily basis using a soil water balance prediction model. In addition, real-time hourly logging of soil moisture in the maize field showed a zone where poorly drained soil remained wetter than predicted. Variable-rate irrigation (VRI) scenarios are presented and compared with uniform-rate irrigation scenarios for 3 years of climate data at these two sites. The results show that implementation of VRI would enable significant potential mean annual water saving (21.8% at Site 1; 26.3% at Site 2). Daily soil water status mapping could be used to control a variable rate irrigator.

A possible mechanism for soil moisture bimodality in humid‐land environments

The origin and causes for the existence of two distinct steady‐state modes of soil moisture probability distribution (pdf) have been attributed to several processes, such as land‐atmosphere feedbacks or shifts in climatic conditions within seasons. Here we argue that the interaction between saturated and unsaturated zones in soils with shallow water tables might represent a possible mechanism leading to such bimodality. This conclusion is achieved by analyzing soil water content measurements in vegetated soil columns artificially constructed in a laboratory. We used these observations to develop a stochastic model for the daily soil water balance, which shows how the interplay between the water table and the unsaturated zone is able to induce soil moisture bimodality.

Statistical spring wheat yield forecasting for the Canadian prairie provinces

A study to forecast regional spring wheat ( Triticum aestivum L.) yields on the Canadian Prairies was conducted, based on simulated daily water use and soil water contents derived from the National Drought Model. Empirical linear regression models were calibrated from 1976 to 2006 spring wheat yield data for this purpose. Potential predictors assessed were mainly those indicators related to water stress conditions at different crop growth stages. Stepwise regression and cross-validation were employed for the selection of the predictors in multivariate linear regression models used for forecasting spring wheat yields from seeding to harvest. The cross-validated “forecasts” for 1976–2006, using data up to harvest, explained 77%, 64%, 63% and 70% of yield variances, respectively, for Alberta, Saskatchewan, Manitoba and the entire Prairie region. Root mean squared error of the “forecasts” ranged from 8% to 11% of the average yields. The prediction accuracy earlier in the season was often lower than later in the season. Usable prediction accuracy was found by the middle of the growing season (around heading or anthesis), but only marginally effective at seeding time, especially so for Saskatchewan.

Combined analysis of energy and water balances to estimate latent heat flux of a sudanian small catchment

Actual evapotranspiration is one of the major components of both energy and water budgets, but is often difficult to monitor over long period with sufficient accuracy. Within the framework of the “AMMA-CATCH” program, a project dedicated to the study of the West African Monsoon, a large aperture scintillometer has been installed in a small catchment (12 km 2 ), located in the North of Benin, a region exposed to sudanian climate. The present study is an attempt to estimate the latent heat flux over this small but heterogeneous catchment based on scintillation and ground observations. The analysis covers the end of the dry season (lasting from February to April 2006). During this period two isolated rainfall events occurred, giving a unique opportunity to study energy and water budgets simultaneously. The comparison between the average sensible heat flux derived from scintillometer observations and the one obtained with conventional eddy correlation shows a relatively good agreement, where the scattering is mainly explained by differences in footprint associated with both instruments. A relevant hourly residual latent heat flux is then obtained through the energy balance equation, with careful attention brought to the net radiation, and the ground heat fluxes. The residual of the energy budget equation is compared to soil water losses from vadose zone and water table, in order to evaluate whether this estimation is consistent with the water budget of the ground. Daily soil water depletion within the first meter of the surface shows a similar dynamic as the one calculated from the energy balance equation, but exhibits a constant 1 mm/day lag. The excess of actual evapotranspiration is supposed to be explained by water table losses and root extraction by trees. Finally, this study shows how combined energy and water budget analysis can help to better understand water transfers at the watershed scale.

Soil water surplus and ENSO events during the last humid period in Argentine Pampean flatlands

Daily soil water balances are estimated for the period 1951-2006 in the Pampean flatlands. Daily soil water surplus is the major output with their annual trends, being analysed because of their harmful effects on crop harvests and floods. Soil water surplus trends were analysed founding positive trends, although only two stations were statistically significant at probability level of 95%. Differing phases of ENSO – using the multivariate ENSO index (MEI) – have differing impacts on rain and soil moisture conditions. The probabilities of occurrence of predetermined soil water surplus and hence flood risk are calculated and mapped.

Soil water variability in the Weatherley grassland catchment, South Africa: II. Soil water content

Soil water content is a major factor that affects the hydrological response of a hillslope or catchment. It is therefore important to have reliable soil water content data to estimate catchment water yield. Daily soil water content (θ) data was calculated based on weekly measured and other data for the Weatherley grassland catchment in South Africa. A modelling procedure, based on the soil water balance equation and the interpretation of the physical properties of soils was used to calculate daily θ for all 28 sites for the six-year period. A statistical model performance indicated that the mean index of agreement was 0.88, root mean square error (RMSE) was 6.8 mm water per 300 mm soil and mean unsystematic RMSE to total RMSE was 93%. These results indicated that the calculated soil water contents agreed well with the measured values and could therefore be used with reasonable confidence to fill data gaps. The proposed procedure therefore affords the possibility to increase the resolution of irregular measured soil water content data. This would significantly advance the usability of such data, because the influence of rainfall events on soil water content is frequently missed by manual soil water content measurements.

Comparing theoretical irrigation requirement and actual irrigation for citrus in Florida

Florida ranks first in citrus production, with nearly 68% of all U.S. citrus growing in the season 2005–2006. Most of the citrus groves are located from central to south Florida, and agricultural irrigation permitting is regulated by three of Florida's five water management districts. Most of the permitting for citrus production in Highlands, Polk and Hillsborough counties is conducted by the Southwest Florida Water Management District (SWFWMD), and quantities are based on the District's AGMOD computer program. In 2003, the SWFWMD implemented new permit criteria so that permitted amounts were more representative of actual water use. This paper compares grower reported citrus irrigation water use in Highlands, Polk and Hillsborough counties from 1994 through 2005 with permitted and theoretical irrigation requirements calculated by a daily water balance. Two different sets of crop coefficients ( K c's) developed for citrus in Florida were compared in the daily soil water balance calculation of theoretical irrigation requirements. The percentage of irrigated area considered in this study ranged from 40 to 60% to simulate a range of grower practices. Meteorological data from two weather stations and additional rainfall information from 50 locations within the three counties was used in the water balance. Missing and error values in the meteorological historical record data were filled with weather generators. The multiannual average water consumption (including cold protection water use) from growers ranged from 243 (Hillsborough) to 406 mm (Highlands) and the multiannual average permitted irrigation requirement (without cold protection) ranged from 295 to 557 mm. The simulated gross irrigation requirements under different scenarios of location- K c-wetted area were variable but mostly lower than the limits established by the district, except for some scenarios in Polk County, whose maximum simulated irrigation value reached 578 mm year −1. In general, permitted limits recommended by the SWFWMD seem to be reasonable for the actual water use by growers in these counties.

Funções de distribuição de probabilidade decendial e mensal para a deficiência hídrica no solo

A determinação das probabilidades de ocorrência de certos níveis de deficiência hídrica do solo (DHS) é fundamental para o planejamento agropecuário e a gestão ambiental. O objetivo deste trabalho foi determinar as funções de distribuição de probabilidades (fdp) que melhor caracterizam a distribuição dos valores de deficiência hídrica no solo. As DHSs decendiais e mensais foram determinadas por meio de balanço hídrico diário do solo para as diferentes capacidades de armazenamento de água disponível dos solos da região de abrangência da estação meteorológica de Santa Maria. Verificou-se que os valores mensais e decendiais de DHS se ajustam às funções de distribuição de probabilidades lognormal, gama, exponencial e Weibull. Utilizando-se as fdp selecionadas para cada período do ano, é possível determinar a probabilidade de ocorrência de qualquer valor de DHS para os solos da região de abrangência da estação meteorológica de Santa Maria.

Drought during canopy development has lasting effect on annual carbon balance in a deciduous temperate forest

* Climate change projections predict an intensifying hydrologic cycle and an increasing frequency of droughts, yet quantitative understanding of the effects on ecosystem carbon exchange remains limited. * Here, the effect of contrasting precipitation and soil moisture dynamics were evaluated on forest carbon exchange using 2 yr of eddy covariance and microclimate data from a 50-yr-old mixed oak woodland in northern Ohio, USA. * The stand accumulated 40% less carbon in a year with drought between bud-break and full leaf expansion (354 +/- 81 g C m(-2) yr(-1) in 2004 and 252 +/- 45 g C m(-2) yr(-1) in 2005). This was caused by greater suppression of gross ecosystem productivity (GEP; 16% = 200 g) than of ecosystem respiration (ER; 11% = 100 g) by drought. Suppressed GEP was traced to lower leaf area, lower apparent quantum yield and lower canopy conductance. The moisture sensitivity of ER may have been mediated by GEP. * The results highlight the vulnerability of the ecosystem to even a moderate drought, when it affects a critical aspect of development. Although the drought was preceded by rain, the storage capacity of the soil seemed limited to 1-2 wk, and therefore droughts longer than this are likely to impair productivity in the region.