Potential Evaporation Rates Research Articles

Drying front and water content dynamics during evaporation from sand delineated by neutron radiography

Evaporative drying of porous media is jointly controlled by external (atmospheric) conditions and by media internal transport properties. Effects of different atmospheric potential evaporative demand on observed drying rates were studied in a series of laboratory experiments using sand‐filled Hele‐Shaw cells. We examined two potential evaporation rates of about 8 and 40 mm per day. The evolution and geometry of the drying front (marking the interface between saturated and partially dry regions) and water content distribution above the drying front were measured every 5 min at 0.1 mm spatial resolution using neutron radiography. Water loss rates decreased with time for both rates, but the decrease was more pronounced for high evaporative demand. External evaporative demand had no effect on drying front geometry or spatial water content distribution for depths below 2 mm. Cycles of roughening and smoothing of drying fronts due to interfacial pinning and unpinning were observed. The water content above the front showed irregular patterns due to formation of isolated liquid clusters with the general profile showing a decrease in mean water content with deepening drying front. Measured water content profiles support the hypothesis that liquid flow supply surface evaporation during stage 1 and water content distribution were not affected by external drying rates. Additionally, observed saturation profiles indicate that the corresponding hydraulic conductivity supports fluxes larger than the highest drying rate measured for sand, suggesting that decreasing drying rate was limited by vapor exchange between progressively drying surface and the viscous boundary layer above.

Transport routes and source regions of Asian dust observed in Korea during the past 40 years (1965–2004)

This paper investigates regional sources and transport routes of Asian dust observed in Korea over the past 40 years, that is from 1965 to 2004. These long-term changes were studied in relation to climatic changes and causes. About 87% of dust events in Korea occur in the spring and have been increasing steadily since the 1980s. These trends can be attributed to regional changes in dust sources to windward and the resultant transport pathways of dust storms. This study establishes that dust events have greatly increased during the 3 years from 2000 to 2002. It may be the result of not only an eastward shift in the regional source of dust but also changes in upper-level synoptic transport patterns. Intensified atmospheric circulation and an associated increase in wind speed as well as changes in snow cover, surface wetness and potential evaporation rates throughout Inner Mongolia and Manchuria are likely to be the main causes of the steadily increasing number of dusty days in Korea.

Phenomenological models of hydrologic processes in south Florida

Summary The objectives of this study were to use existing hydrologic data to identify phenomenological models appropriate for describing key hydrologic processes in south Florida, and collect field data to develop improved process equations. Analyses of 3300 storm events within a 250-km 2 study area in south Florida indicate that rainfall amounts and durations are typically correlated over distances on the order of 2 km. For 1-h duration storms, significant correlation exists for length scales on the order of 1 km, and for 24-h duration storms, significant correlation exists for length scales on the order of 10 km. Double-ring infiltrometer tests indicate that asymptotic infiltration capacities can be reasonably estimated from soil texture. Infiltration capacities are characterized by relatively rapid decay rates and typically approach a constant value within 10 minutes of ponding. Rainfall-recharge data indicate a linear relation between rainfall and recharge characterized by a specific yield of 0.26 and a threshold rainfall of 1 cm. Evaporation from the saturated zone during persistent dry periods can be described by a linear model in which the saturated-zone evaporation rate is equal to the potential evaporation rate for a depth of 1.4 m and decreases to zero at a depth of 2.5 m.

Regression method for estimating rainfall recharge at unconfined sandy aquifers with an equatorial climate

AbstractA reliable estimate of rainfall recharge is essential for groundwater system managements. This study develops a method based on regression equations for estimating rainfall recharge at unconfined sandy aquifers with an equatorial climate. The developed method (GR‐I method) is generally efficient for estimating long‐term regional recharge, as the computational procedures could be formulated and executed easily using Microsoft's Excel spreadsheet. More importantly, its application could be extended to sand textures different from the sand texture used in developing the regression equations. To evaluate its reliability, the method was applied to estimate monthly gross recharge percentages at the Changi reclaimed land. When ignoring the effect of rainfall clusters, the GR‐I method was found to underestimate the monthly gross recharge percentages for those months with high monthly rainfall depths. By integrating the effect of rainfall clusters, the GR‐I method yields reliable estimates of monthly gross recharge percentages. By including daily potential evaporation as an additional input variable, the Extended GR‐I method was found to be not superior to the GR‐I method, implying that soil moisture availability is the major governing factor for actual soil evaporation in the highly porous sand medium, instead of atmospheric demand represented by the potential evaporation rate. Using the GR‐I method, the mean annual net recharge percentage of the study site was found to fall between 56·9 and 69·9%, which corresponds to a net recharge depth of 1073·8–1745·8 mm. Although the developed method provides a good alternative to other widely used methods, its recharge estimates still needs to be collaborated with estimates from other methods, as multiple techniques are highly recommended in any groundwater recharge estimations. Copyright © 2007 John Wiley & Sons, Ltd.

Saltcedar Water Use: Realistic and Unrealistic Expectations

Saltcedar (Tamarix spp.) is a widespread invasive plant found in riparian corridors and floodplains in 16 western states. In addition to being associated with such problems as increased soil salinity and decreased plant diversity, saltcedar has been reported to be a prolific water user. Popular press articles widely report that each individual saltcedar tree can use as much as 757 L (200 gallons) per day. Consequently massive control and removal efforts are underway to reduce transpirational water loss and increase water salvage for arid and semiarid environments. Although the potential economic benefits of these control efforts are touted, it has not been proven whether such water savings are possible on a stream level. The original citation for the 757-L estimate does not list the experimental design or techniques used to arrive at this value. We use three lines of evidence—peer-reviewed scientific literature, sap flux rates and sap wood area, and potential evaporation rates—to demonstrate the improbability that saltcedar, or any other woody species, can use this much water per tree on a daily basis. A more realistic estimate of maximum tree-level daily water use derived from sap flux measurements would be < 122 L · d−1 (32.2 gallons). Estimates of water salvage would be grossly overestimated using the popular water use value (757 L · d−1), and economic benefits from saltcedar control based solely on water salvage are questionable.

Monitoring water stress using time series of observed to unstressed surface temperature difference

Remote sensing data in the thermal infra red (TIR) part of the spectrum provides indirect estimates of water stress – defined as a function of the ratio between actual and potential evaporation rates – at the earth surface. During the first stage of evaporation (“energy limited” evaporation), this ratio is close to one. During the second stage of evaporation (“soil controlled” evaporation) water stress occurs and as a result this ratio drops below one. Recently, methods using TIR data to monitor stress have shifted from establishing empirical relationships between combined vegetation cover/temperature indices and soil moisture status to data assimilation of surface temperature into complex soil–vegetation–atmosphere transfer models. However, data and expertise are often lacking to widely apply those methods. In this paper we investigate the proof-of-concept of using solely the difference between actual and unstressed surface temperature as a baseline to monitor water stress. The unstressed temperature is the equilibrium temperature of a given surface expressed in potential conditions, computed with an energy balance model. Theoretical, modelling, and experimental documentation of the proof-of-concept are shown for datasets acquired within the frame of two international experiments in semi-arid region. We show that the difference between the observed and the unstressed surface temperatures is almost linearly related to water stress. A sensitivity study is carried out to test the impact of modelling errors on the evaluation of the unstressed temperature. We found that even with inaccurate but realistic values of the surface parameters used to solve the energy balance and compute the unstressed temperature, the observed to unstressed surface temperature difference is still more relevant to detect second-stage processes than the difference between the observed surface temperature and the air temperature. The perspective of using an empirical index based on this difference is also investigated. These results are especially attractive for application based on TIR satellite imagery at a regional scale.

The effect of mulching and tillage on the water and temperature regimes of a loess soil: Experimental findings and modeling

Crop residues and tillage are being advocated for their potential effectiveness to modify the soil hydrothermal regime. This study was carried out to quantify the effect of straw mulching and rotary hoeing on the soil water and thermal regimes of a loess soil. The field experiment consisted of four treatments: (1) no mulching and no rotary hoeing as control, (2) rotary hoeing, (3) wheat straw mulching, and (4) wheat straw mulching with rotary hoeing. During the study period from 5 August to 20 September 2002, soil water content and pressure head were measured daily at five soil depths (0.05, 0.15, 0.30, 0.45 and 0.60 m). Soil temperatures were measured at hourly resolution at three depths (0.05, 0.15 and 0.30 m). Mulching decreased soil water loss on an average by 0.39 mm d −1 and rotary hoeing increased water loss on an average by 0.12 mm d −1 as compared to control. Volumetric soil water contents at pF 1, 1.8 and 2.5 up to 30 cm depth were highest (0.418, 0.390, and 0.360 m 3 m −3, respectively) with the application of wheat straw mulch and lowest (0.393, 0.363, and 0.333 m 3 m −3, respectively) with the rotary hoeing. Soil thermal conductivity measured at pF 1, 1.8, 2.5, 3, and 3.7 decreased with increasing suctions in all the treatments. However, the tillage and mulching did not affect the soil thermal conductivity. Further, compared with the control, mulching reduced average soil temperatures by 0.74, 0.66, 0.58 °C at 0.05, 0.15, and 0.30 m, respectively, during the study period. The rotary hoeing tillage slightly increased the average soil temperature by 0.21 °C at 0.05 m depth compared to control. The tillage effect did not transmit to deeper depths. The numerical model Hydrus-1D was used to simulate the water and temperature regimes of the treatments. Simulations with hydraulic parameters derived from laboratory measurements did not yield satisfactory results. Only when the hydraulic parameters were optimized by the inverse method, simulations performed well. The largest deviations were observed in the wheat straw mulching treatment. Simulations were further improved by adjusting the potential evaporation rate from the measured data which was achieved by linking the inversion code UCODE to the Hydrus-1D. Soil temperatures at 0.05 and 0.15 m in all the treatments were modeled well, yielding root mean square errors between 0.3 and 1.7 °C. As for soil water, the largest temperature deviations were found for the mulching treatment. All simulations underestimated soil temperatures at 0.30 m. In conclusion, crop residue can be utilized as mulching to improve the soil hydrothermal regime and the Hydrus-1D model can be used as a tool for analyzing water and heat transport processes and for estimating hydraulic transport parameters under field conditions.

Effects of meteorological and hydrogeological factors on gross recharge percentage at unconfined sandy aquifers with an equatorial climate

AbstractA thorough understanding of rainfall recharge processes and their controlling factors is essential for management of groundwater systems. This study investigates the effects of various meteorological and hydrogeological factors on the gross recharge percentages, the rainfall–recharge relationships and the recharge threshold values for unconfined sandy aquifers under an equatorial climate. Among the meteorological factors investigated, rainfall intensity was found to have the most significant impact on the gross recharge rate. The effects of potential evaporation rate, relative humidity and air temperature on the gross recharge percentage were significant when the vadose zone thickness is larger than 2·5 m. The recharge threshold values were found to depend strongly on the vadose zone thickness. The rainfall–recharge relationships could generally be well defined by a normal–log relationship. The rainfall–recharge relationships derived here are applicable to yield estimates of gross recharge percentages for unconfined sandy aquifers under an equatorial climate, using rainfall intensity and vadose zone thickness as input variables. In this study, a theory was developed and validated to provide physical explanations for the observations, based on the residence time of the percolated rainwater within the vadose zone. Among the soil hydraulic parameters tested, porosity and saturated hydraulic conductivity were found to have the most pronounced effects on the gross recharge percentage. Utilizing the sensitivity results and the theory derived, an approach was developed for extending the application of the derived rainfall–recharge relationships to other sand textures. The approach was found to be capable of producing rough and fast estimations of gross recharge percentage for other sand textures. Copyright © 2007 John Wiley &amp; Sons, Ltd.

Lychee tree parameters for water balance modeling

Lychee (Litchi chinensis Sonn.) is widely grown under irrigation in the tropical northern Thailand highlands. Water efficient irrigation requires sound irrigation scheduling for which the requisite soil moisture information can be obtained from water balance modeling. A prerequisite for water balance predictions are plant parameters which describe interception, root distribution evaporation and transpiration. These parameters depend on climate, soil, as well as plant physiology, variety and age. This study investigated the plant parameters for 7-year-old lychee trees in tropical granite landscapes, as they are as yet unavailable. Interception could be satisfactorily predicted with the modified Gash model. The Gash parameters (canopy capacity per canopy cover area (Sc), canopy cover factor (c)) were determined to be 21.8 mm and 0.22, respectively. The spatial distribution of lychee tree roots depended on slope inclination. On the level plateau position, root length density (RLD) distribution was trunk-symmetrical and could be predicted with an empirical function. On the slope, the asymmetrical and irregular root development was not predictable. The suitability of the dual crop coefficient approach predicting daily potential evaporation (Epot) and daily potential transpiration (Tpot) rates for water balance modeling was limited due to the weak correlations of Epot and Tpot with the potential reference evapotranspiration (ETo). As a result, no universal values for the potential evaporation coefficient (Ke,max) and the potential transpiration coefficient (Kcb) could be determined. Hence, Epot and Tpot measurements are mandatory if accurate Epot and Tpot data are necessary. In the case of missing measurements, Ke,max values of 0.6 and 1.6 are recommended for rough Epot estimates underneath and in-between the lychee tree canopies. For Tpot predictions in irrigation scheduling, a relatively high Kcb of 0.8 is recommended in order to ensure a water stress free fruit development within the irrigation season.

Assessment of drought vulnerability based on the soil moisture PDF

This paper studies the statistics of the soil moisture condition and its monthly variation for the purpose of evaluating drought vulnerability. A zero-dimensional soil moisture dynamics model with the rainfall forcing by the rectangular pulses Poisson process model are used to simulate the soil moisture time series for three sites in Korea: Seoul, Daegu, and Jeonju. These sites are located in the central, south-eastern, and south-western parts of the Korean Peninsular, respectively. The model parameters are estimated on a monthly basis using hourly rainfall data and monthly potential evaporation rates obtained by the Penmann method. The resulting soil moisture simulations are summarized on a monthly basis. In brief, the conclusions of our study are as follows. (1) Strong seasonality is observed in the simulations of soil moisture. The soil moisture mean is less than 0.5 during the dry spring season (March, April, and June), but other months exceed the 0.5 value. (2) The spring season is characterized by a low mean value, a high standard deviation and a positive skewness of the soil moisture content. On the other hand, the wet season is characterized by a high mean value, low standard deviation, and negative skewness of the soil moisture content. Thus, in the spring season, much drier soil moisture conditions are apparent due to the higher variability and positive skewness of the soil moisture probability density function (PDF), which also indicates more vulnerability to severe drought occurrence. (3) Seoul, Daegue, and Jeonju show very similar overall trends of soil moisture variation; however, Daegue shows the least soil moisture contents all through the year, which implies that the south-eastern part of the Korean Peninsula is most vulnerable to drought. On the other hand, the central part and the south-western part of the Korean peninsula are found to be less vulnerable to the risk of drought. The conclusions of the study are in agreement with the climatology of the Korean Peninsula.

Theoretical analysis of a remotely measurable soil evaporation transfer coefficient

Reducing soil evaporation ( E) is an important way to increase water use efficiency for agriculture and sustainable water use. To achieve this goal, an accurate estimate of soil evaporation at the macro-scale level is necessary. By including the surface temperature of dry soil, the three temperatures model (3T model) was proposed as an estimate of E, and its temperature term was defined as the soil evaporation transfer coefficient ( h a). Although it has been primarily shown that h a determines the boundaries of E, further studies of its properties are necessary because h a has the application potential for remote sensing. The objectives of this study are to (1) investigate the relationships between h a and those parameters that are important for E estimation but are not able to be remotely measured (aerodynamic resistance, soil surface resistance ( r s), and cumulative evaporation ( E c)); and (2) discuss the possibility for remote sensing application. Two experiments were carried out for these purposes: one in an open field and the other one in a closed growth chamber at constant temperature, humidity, and radiation. The following results were obtained: (1) Given the boundaries of 0 ≤ h a ≤ 1, E was determined between its maximum value (potential evaporation rate) and minimum value (zero); (2) During the period when evaporation is controlled by r s, a linear relation between h a and log ( r s) is observed with a coefficient of determination r 2 = 0.76. Because h a and r s are independent variables with significant differences in properties and magnitudes, these results indicate that h a and r s are well related to each other; (3) During stage 1 of evaporation, cumulative evaporation ( E c ) increased with time while h a remained constant. Afterwards, h a linearly increased with E c. The relationships among h a, E c, r a, and r s would be useful for estimation of evapotranspiration by remote sensing.

Responses of potatoes ( Solanum tuberosum L.) to irrigation and nitrogen in a hot, dry climate : I. Water use

The effects of differential irrigation and fertiliser treatments on the water use of potatoes ( Solanum tuberosum L. cv. Desirée) were studied over 2 years in the hot dry climate of northeast Portugal. Total actual evapotranspiration (ET c) ranged from 150 to 320 mm in 1988, and from 190 to 550 mm in 1989 depending mainly on irrigation treatment, potential evaporation rates (ET p) and duration of the growing season. By comparison, the effects of nitrogen fertiliser on total water use were relatively small. Although nitrogen increased transpiration (larger leaf canopy), it reduced evaporation from the soil surface, in frequently irrigated plots, by similar amounts. As a result, in well-irrigated crops, the ET c/ET p ratio averaged 0.85 over the season, regardless of nitrogen level. Evaporation from the soil surface represented 15–25% of total water use by well-fertilised plants, but as much as 30–50% from the sparse stands of unfertilised crops. The proportion of water extracted from each depth increment of the silt-loam soil declined logarithmically, from the surface to 1.1 m depth, the maximum measured, for irrigated crops, and linearly when rain-fed. The ET c/ET p ratio fell below unity when 25–30% of the available water in the top metre had been depleted, equivalent to soil water deficits (SWDs) of 45–50 mm. By comparison, ET c declined to zero when 75–90% of the available water had been extracted, corresponding to actual deficits of 135–150 mm. Peak ET c rates reached 12–13 mm per day on days immediately following irrigation, nearly twice ET p (possibly due to the influence of advection) but then declined logarithmically with time to about 3 mm per day within 5 days. Using the same data, a companion paper reports the influence of climatic conditions on the yield responses to water of potato crops grown in the region.

Soil Water Accumulation under Different Precipitation, Potential Evaporation, and Straw Mulch Conditions

Small precipitation amounts generally have low effectiveness for crop production in semiarid regions. Our objective was to determine potential evaporation (PE; 3, 6, or 12 mm d−1) and straw‐mulch (0, 2, or 4 Mg ha−1) rate effects on water accumulation in Pullman (Torrertic Paleustoll, 37% clay) and Randall (Ustic Epiaquerts, 57% clay) soils when small amounts of water (simulated precipitation; 5, 10, or 20 mm) were applied. Water accumulation was affected in order by water‐application amount &gt; PE &gt; mulch &gt; soil clay content. Mulching at 2.0 and 4.0 Mg ha−1 increased storage efficiency of 5‐mm water applications by &gt;60 and 100%, respectively, in both soils when PE was 3 mm d−1 With 5‐mm water applications and 6 mm d−1 PE, &gt;10% of applied water was stored in mulched soils, but not in bare soils. When PE was 12 mm d−1, little storage from 5‐mm applications occurred in bare soils, but 3 to 6% storage occurred when the mulch rate was 4.0 Mg ha−1 To obtain &gt;10% water storage when the PE rate was 12 mm d−1, 10‐mm water applications and a 2 Mg ha−1 mulch rate were necessary. Evaporation rates were slightly higher for mulched soil than for bare soil in the late stage. Soil clay contents were correlated positively with accumulative evaporation in the late stage. Soil wetting depth increased with increases in mulch rates. Based on this study, straw mulching has potential for increasing soil water storage from small amounts of precipitation.

The energy and water balance of high-latitude wetlands: controls and extrapolation.

This paper examines, with examples, controls on the energy and water balance of northern wetlands. Most wetlands have organic soils and are thus peatlands. High-latitude wetlands are underlain by ice-rich permafrost, which helps maintain wetland systems and also imparts special characteristics to their energy and water balances. In North America, components of the radiation balance decrease linearly poleward, whereas the poleward rate of decrease of temperature and precipitation lessens. During the four-month summer of a high subarctic wetland, net radiation is large and the latent heat flux dominates the energy cycle. The ground heat flux is substantial, especially in early summer, when the ice-rich ground is rapidly thawing. Winter begins in October and heat loss from the ground approximately balances negative net radiation. The summer energy and water balance differs among terrain units. Large shallow lakes exhibit larger net radiation and potential evaporation rates than surrounding wetland surfaces which, in turn, exhibit substantially larger magnitudes than dryland terrain. There is a variable withdrawal rate of soil moisture depending on soil types and plant rooting characteristics, which influences the actual evaporation from the surface. Synoptic weather systems play a major role in day-to-day energy and water responses to climate forcing. Long-term modelling of the water balance of a wetland shows year-to-year persistence in climatic patterns. Although net radiation, temperature and precipitation all influence the magnitudes of water deficit, the precipitation inputs are of paramount importance. Our ability to fully understand, model and extrapolate, in space and time, the major controls on the surface climate of wetlands, is evaluated. Spatial extrapolation is seen to be more readily achieved than temporal extrapolation.

Simulating mountain runoff with meso-scale weather model rainfall estimates: a New Zealand experience

During October 1996 a series of intensive meteorological measurements were made along the 500 km length of the Southern Alps of New Zealand. These measurements were made to investigate the physical processes responsible for producing heavy rainfall and to evaluate the performance of a meso-scale weather forecast model. A by-product of the work was sets of simulated hourly rainfalls over the entire mountain range. Rainfall estimates were made every 24 h on a 20×20 km resolution grid covering the whole of New Zealand. Successive sets of 24-hourly values were combined to produce a continuous 29-day sequence of model generated rainfalls. The study region covers remote mountain river basins in which there are few rainfall data but over 20 continuously recording river flow monitoring stations. For many of the basins, the runoff rate is an order of magnitude larger than the potential evaporation rate. To a first approximation, and over time periods of days, the river catchments act like large rain gauges. For each basin a rainfall–runoff model was built using the Topmodel assumptions that saturated hydraulic conductivity decreases exponentially with depth from the ground surface, the hydraulic gradient of the saturated zone is equal to the topographic gradient, and subsurface recharge is uniform. As the water table rises in response to rainfall over each sub-basin, increasing amounts of the ground surface become saturated, and rainfall falling directly onto these saturated areas generates much of the storm runoff. Results are presented for basins ranging in area from 12 to 3830 km 2 and that lie on both the windward and leeward sides of the mountain range.

Development and evaluation of a soil water evaporation model to assess the effects of soil texture, tillage and crop residue management under field conditions

Abstract. A number of mathematical models to predict soil water evaporation are available in the literature which generally require complex input data. In the present study, a simple parametric model has been developed by coupling existing and newly developed equations to assess soil water evaporation and drainage under field conditions in relation to potential evaporation rate, soil texture, time and depth of tillage and crop residue management. The model has moderate input data requirements and predicts well the effects of tillage and crop residue management practices on soil water loss (evaporation+drainage) with multi‐drying and ‐wetting cycles prevailing under natural conditions. The root mean squares of deviations between observed and predicted cumulative water loss at different periods of study were 0.82, 2.04, 2.31 and 1.74 cm for untreated, residue‐mulch, tillage and residue‐incorporated treatments, respectively. Simulation analysis on cumulative evaporation and evaporation rate has shown that the evaporation reduction with different combinations of tillage and crop residue followed the order of residue‐undercut&gt;residue‐mulch&gt;residue‐incorporated&gt;tillage. Thus, the magnitude of beneficial effects of crop residues and tillage on soil water evaporation reduction are associated with amount of residues, mode of residue management (mulched or incorporated in the soil) and time and depth of tillage.

Influence of temporal variability of rainfall on interception loss. Part I. Point analysis

An interception model that links the temporal variability of rainfall with the storm-based description of the interception process is derived. Analytical formulae for long-term interception loss are obtained for precipitation with statistical characteristics derived from observations. The analysis of the results indicates that the point interception loss is controlled primarily by three time scales: the mean inter-storm arrival time τ a, the mean storm duration τ r and the time to evaporate a saturated canopy τ 0 which depends on canopy water holding capacity W c and the wet canopy potential evaporation rate E I0, and less importantly, on storm intensity. Additional assumption of rainfall stationarity leads to a relation between long-term interception loss and gross rainfall that requires a very small amount of input data. The interception loss predicted by the analytical model agrees well with that of a Rutter model driven by a synthetic rainfall time series with the same statistics. Using the parameter values estimated from the observed rainfall data in the Amazon and southwestern France, the analytical results predict a long-term interception loss close to that observed.

乾燥裸地面からの蒸発の合理的パラメタリゼーション

The evaporation of water from bare soil changes in mechanism as well as in magnitude as the surface dries. This evaporation proceeds in three stages, the third of which has a mechanism completely different from the other two stages. Therefore, only the parameterization schemes for soil-surface evaporation that take account of this stage switching should be considered as rational. A rational parameterization of the soil-surface evaporation based on a three-layer model of the vertical distribution of soil moisture is proposed. Each stage of evaporation is characterized by one of the three soil layers that comes to the surface. The switching from one stage to another can be determined from the difference between surface soil temperature and screen-level air temperature. The evaporation rate in the first stage is equal to the potential evaporation rate E p , and that in the second stage can be expressed by the product of E p and surface moisture availability M, which depends on the wetness of the superficial layer of soil and the soil type. Since these are practically the same as the existing parameterizations, only a general idea of formulating the two stages is given in this paper. The third-stage process, however, is formulated by a newly invented scheme that will be called the DSL bulk method. An example of the application of this method to the Tottori-Dune sand is also shown. This parameterization makes it possible to evaluate the evaporation rate in the third stage by the surface soil temperature and the average water content for the top 5 cm of soil, and both can be measured by remote sensing.

A model to consider the spatial variability of rainfall partitioning within deciduous canopy. II. Model parameterisation and testing

This paper gives details of how the spatially variable rainfall partitioning model, presented in Davie and Durocher, (1997) [Hydrol. Process., 11, 1509–1523], is parameterized, and the testing of its capabilities. A majority of the input parameters required for running the model can be derived from physical measurements. The remainder can be derived from previous forest interception studies. The measured values used in model testing were from a mixed oak and sweet chestnut plantation on the outskirts of Bristol, UK. A sensitivity analysis of the model has been carried out. The importance to throughfall of potential evaporation rate, storage capacity and gap area within the canopy is highlighted. For stemflow the proportion of tree crown area generating stemflow is the most important parameter. This is largely a reflection of the model structure. The model has been tested with field data to evaluate its ability for predicting: total throughfall and stemflow for individual events; spatial variability in rainfall partitioning; and temporal dynamics of throughfall. The model was able reproduce these data sets; however the field data (from an intensive plot study) was not adequate to allow complete validation of the model. The novel feature of the model (spatial variability of rainfall partitioning) was shown to be quite well simulated. Suggestions are given for using this type of model in future research. © 1997 John Wiley & Sons, Ltd.

Evaporation from an eastern Siberian larch forest

Total forest evaporation ( λE), understorey evaporation, and environmental variables were measured on nine summer days under different weather conditions in a 130-year-old stand of Larix gmelinii (Rupr.) Rupr. trees located 160 km south of Yakutsk in eastern Siberia, Russia (61°N, 128°E, 300m above sea-level (a.s.l.)). Tree and broad-leaved understorey vegetation one-sided leaf area indices were 1.5 and 1.0, respectively. Agreement of λE and sensible heat flux ( H), both measured by eddy covariance, and the available energy ( R a) was generally good: ( H + λE) = 0.83 R a + 9 W m −2 with r 2 = 0.92 for 364 half-hour periods and the mean ± 95% confidence limit was 129 ± 17 for ( H + λE) and 144 ± 19 for R a. Daily E was 1.6–2.2 min, less than half of the potential evaporation rate and accounting for 31–50% of R a, with the lowest percentage on clear days. A perusal of the sparse literature revealed that average daily E of boreal coniferous forest during the tree growing season (1.9 mm day −1 for this study) is relatively conservative, suggesting that low evaporation rates are a feature of this biome's energy balance. Using the Penman-Monteith equation, the maximum bulk-surface conductance ( G smax) was 10 mm s −1. E and G s were regulated by irradiance, air saturation deficit, and surface soil water content during a week-long dry period following 20 mm rainfall. From lysimeter measurements, 50% of E emanated from the understorey at a rate proportional to R a. Based on the measurements and published climatological data, including average annual precipitation equal to 213 mm, water balance calculations indicated growing season forest E equal to 169 mm, the occurrence of a late summer-autumn soil water deficit, and annual runoff of 44 mm by snowmelt.