Initial Soil Water Content Research Articles

Evaluation of environmental factors and equipment configuration on efficiency of a solar soil water collector

The aim of this paper is to assess the effects of environmental factors and equipment configuration on the recovery of soil water using a solar collector. The recovery of water from soil is difficult, since the energy requirements are very high. Solar energy provides a solution to this problem, especially in countries with high solar radiation. In the current study, it was found that the amount of water collected depended largely upon the air temperature and soil water content. In winter, 0.465 L m−2 day−1 were collected compared to 1.163 L m−2 day−1 in summer. The initial soil water content was equally important; dry soil tended to yield less condensate than wet soil. Saturated soils yielded 0.814 L m−2 day−1 in winter compared to 1.395 L m−2 day−1 in summer. The type of soil (i.e. gravel, silt and loamy sand) affected the release of water from the surface, as a result of differences in soil water release characteristics. The air space between the soil surface and the condensate collecting surface also had an effect on condensate collection; the larger this distance, the greater the amount of condensate that was collected. The configuration of the condensate collecting surface also had an effect on the amount of condensate obtained; the shallower the slope of the surface, the greater the amount of condensate that was collected. The collector was most effective in summer, on cloudless days, with wet poorly-drained silt soils.

Effect of the initial soil moisture content on the spatial distribution of the water retention

The initial soil moisture content affects the water flow and solute transport through the vadose zone, but researchers are in disagreement about the extent and nature of its effects. Better understanding of the initial moisture effect on the water movement will help to prevent groundwater contamination and increase crop production by improving the efficiency of water use in irrigation practice. Therefore, in this study, the effect of the initial moisture content on the spatial distribution of the water retention was investigated in the field. A total of 4 cm of water was applied to duplicate plots with each of three initial moisture conditions within 2 h using a rainfall simulator. Following the application and a 2-h redistribution period, 100 soil samples were taken from different depths of each plot using a grid sampling system to be analyzed for their gravimetric water content in the laboratory. Statistical and geostatistical analyses were performed to analyze the spatial structure of the collected data. The results showed that the preferential flow was more evident in the case of the dry initial soil water content than for the two wetter initial conditions. Both the classical and geostatistical analyses supported that the overall water retention was uniformly distributed throughout the profile except at 20–30 cm, where the coefficient of variation and the percent nugget to total semivariance ratio were high, indicating some degree of preferential flow through large pores (macropores). These results suggest that similar studies should be conducted on different field soils under more different initial moisture conditions so that the effect of the macropores on the water flow and chemical transport can be better understood.

Atrazine and Alachlor Transport in Claypan Soils as Influenced by Differential Antecedent Soil Water Content

Increased attention to ground water contamination has encouraged an interest in mechanisms of solute transport through soils. Few studies have investigated the effect of the initial soil water content on the transport and degradation of herbicides for claypan soils. We investigated the effect of claypan soils at initial field capacity vs. permanent wilting level on atrazine and alachlor transport. The soil studied was Mexico silt loam (fine, smectitic, mesic Aeric Vertic Epiaqualf) with a subsoil clay content, primarily montmorillonite, of >40%. Strontium bromide, atrazine, and alachlor were applied to plots; half were at field capacity (Wet treatment), and half were near the permanent wilting point (Dry treatment). Soil cores were removed at selected depths and times, and cores were analyzed for bromide and herbicide concentrations. Bromide, atrazine, and alachlor were detected at the 0.90-m depth in dry plots within 15 d after experiment initiation. Bromide was detected 0.15 m deeper (P < 0.05) in the Dry compared with the Wet treatment at 1, 7, and 60 d after application and >0.30 m deeper (P < 0.01) in the Dry treatment at 15 and 30 d after application; similar treatment results were found for atrazine and alachlor, although on fewer dates with significant differences. The mobility order of the applied chemicals was bromide > atrazine > alachlor. The atrazine apparent half-life was significantly longer in the Dry plots compared with the Wet plots. The retardation factor determined from the relative velocity of each herbicide to that of bromide was higher for alachlor than for atrazine. This study identifies the impact that shrinkage cracks have for different moisture conditions on preferential transport of herbicides in claypan soils.

Green–Ampt Model for Layered Soils with Nonuniform Initial Water Content Under Unsteady Infiltration

The Green–Ampt equation is often used to estimate infiltration in hydrologic modeling. Only a few of the currently used simulation models, however, can simulate infiltration into a heterogeneous soil with nonuniform initial soil water content by the Green–Ampt equation during nonsteady rainfall. In this study, we derived an infiltration model for layered soils under nonsteady infiltration based on the Green–Ampt method (Green–Ampt for Layered Soils, or GALS). In the GALS model, a formula for estimating average suction (S) was derived to include initial soil water effect, and the soil profile was divided into several layers to consider both soil texture and initial soil moisture effects on infiltration. The new model can predict infiltration capacity, ponding time, and cumulative infiltration. Three infiltration experiments, steady‐state infiltration into a layered soil with nonuniform initial soil water content, nonsteady infiltration into a layered soil with uniform soil water content; and nonsteady‐state infiltration into a layered soil with nonuniform soil water content, were conducted for model testing. Comparison showed that the GALS model was in good agreement with another Green–Ampt‐based model and the measured data.

Effectual components on furrow infiltration

AbstractDesign, management and evaluation of furrow irrigation depend on how well infiltration characteristics are identified. Furrow infiltration varies with different variables, and modelling of such phenomena is a complex process at field scale. This research was conducted to quantify the contributions of opportunity time (τ), water flow depth (Dg), initial soil water content (θi), flow section area (A) and wetted perimeter (Wp), as independent variables, on cumulative infiltration (CI) variations using path analysis method; and to develop empirical relationships between field‐wide furrow infiltration and independent variables. Path analysis was used with the aim of scrutinizing direct (DT) and indirect (IDT) effects of five exogenous variables, namely τ, Dg, θi, A and Wp on the endogenous variable of CI. Furrow infiltration was measured by blocked furrow infiltrometers at 56 infiltration sites over a 70 × 130 m field plot. The furrow cross‐sections were taken using a profilometer, whereas flow section area and wetted perimeter were graphically determined from measured cross‐sections. Initial soil water content was obtained by means of the gravimetric method, with drying by the burning alcohol technique in the field. Results revealed that total effects of τ, Dg, θi, A and Wp on furrow infiltration were 0.790, 0.370, −0.359, 0.349 and 0.301; and DT effects of those variables were 92.4, 12.16, 36.50, 42.12 and 14.95% of their total effects, respectively. Consequently, the τ, θi, and A were recognized as the effectual components on the field‐wide furrow infiltration, on the basis of the largest direct effects of the variables. Total and the DT effects of Wp on CI were relatively diminutive compared with other variables. Copyright © 2008 John Wiley &amp; Sons, Ltd.

A MATLAB program for estimation of unsaturated hydraulic soil parameters using an infiltrometer technique

We combined an inverse routine for assessing the hydraulic soil parameters of the Campbell/Mualem model with the power series solution developed by Philip for describing one-dimensional vertical infiltration into a homogenous soil. We based the estimation routine on a proposed measurement procedure especially suitable for early-time infiltrometer experiments where the flow can be considered as one-dimensional. The routine requires input of the initial soil water content and cumulative infiltration in two experiments with different pressures at the upper boundary and/or initial conditions. An independent measurement of the soil water content at saturation may reduce the uncertainty of estimated parameters. Response surfaces of the objective function were analysed. Scenarios for various soils and conditions, using numerically generated synthetic cumulative infiltration data with normally distributed errors, show promising results for finding the true values of the optimized parameters. We also investigated the effects of measurement frequency for the cumulative infiltration and errors in water content determinations.

Using HYDRUS-2D Simulation Model to Evaluate Wetted Soil Volume in Subsurface Drip Irrigation Systems

Drip irrigation is considered one of the most efficient irrigation systems. Alternatively to the traditional drip irrigation systems, laterals can be installed below the soil surface. Realizing the subsurface drip irrigation (SDI), which recently has been increasing in use as a consequence of advances in plastics technology, making SDI equipment more affordable and long lasting. Due to its potential high efficiency SDI may produce benefits, especially in places where water is a limited source. As the use of SDI is relatively new, a better understanding of the infiltration process around a buried point source can contribute to increased water use efficiency and consequently the success of drip irrigation system. In addition, proper design and management of such a system needs the judicious combination of drip spacing, discharge rates, irrigation duration and time interval between consecutive irrigations. To this aim, numerical models can represent a powerful tool to analyze the evolution of the wetting pattern during the distribution and redistribution processes, in order to explore SDI management strategies, to set up the duration of irrigation, and finally to optimize water use efficiency. In the paper the suitability of the HYDRUS-2D simulation model is verified, at the scale of a single emitter, on the basis of experimental observations, with the aim to assess the axis-symmetrical infiltration process consequent to subsurface drip irrigation. The model was then applied in order to evaluate the main dimensions of the wetted soil volume surrounding the emitter during irrigation as a function of time and initial soil water content. The investigation, carried out in a sandy-loam soil, showed the suitability of the model to well simulate infiltration processes around an emitter during irrigation. Model application allowed also, for the examined soil, to evaluate the emitter spacing accounting for the maximum soil depth to irrigate.

Determining soil saturated hydraulic conductivity and sorptivity from single ring infiltration tests

SummaryThe difference between the cumulative infiltration occurring during three‐dimensional axisymmetric and one‐dimensional vertical flow is a linear function of time. The slope of this line is a function of the source radius, initial and final volumetric soil water contents and the soil sorptivity. This allows the determination of the sorptivity and saturated conductivity of the soil from data of axisymmetric flow in a single ring of small diameter under negligible head of water. The method is based on the optimization of the sorptivity and saturated conductivity on the one‐dimensional vertical cumulative infiltration inferred from axisymmetric flow data. To examine the reliability of the method to determine these parameters, numerical three‐ and one‐dimensional data are generated on soils with known hydrologic properties from the literature. The linearity versus time of the difference of the two types of flow is verified. Several physically based expressions for the vertical cumulative infiltration as a function of time are considered. The optimized values of the sorptivity and saturated conductivity are compared to the their real known values. Despite the large errors on the optimized parameters, namely the saturated conductivity, the error on the vertical predicted cumulative infiltration is limited to 10%. This makes possible the application of this method on a large scale for hydrological modelling purposes.

Deep Percolation and its Effects on Shallow Groundwater Level Rise Following Flood Irrigation

Deep percolation (DP) from irrigation may be important for groundwater recharge in irrigated agricultural river corridors of arid regions, yet few studies of this physiographic setting have characterized both percolation and its direct effects on groundwater levels. The objectives of our study in a sandy loam, flood-irrigated, alfalfa-grass field in northern New Mexico were to (1) compare DP below the 1 m effective root zone based on water balance method (WBM) and Root Zone Water Quality Model (RZWQM) simulations, and (2) characterize effects of DP on shallow groundwater levels. Irrigation water applications were metered, and automated instrumentation measured soil water content and climate data for WBM calculations and RZWQM simulations. Groundwater response was characterized by recorded below-field water levels in four experimental wells. DP varied with initial soil water content and water application amount, ranging from 5 to 18 cm (mean 11.2 4.1 SD) with the WBM and from 6 to 17 cm (10.6 3.8 SD) with RZWQM (using 0.0005 cm3 cm-3 macroporosity). Across irrigation events, there was high correlation (r = 0.90) between WBM and RZWQM DP. Peak water level response (up to 38 cm) varied from 8 to 16 h after irrigation onset depending on well location and water application amount. Study results show that flood irrigation is a significant source of shallow groundwater recharge. The high correlation between calculated and simulated deep percolation without iterative model calibration indicates that RZWQM can be a useful tool to estimate DP and extend localized field studies to larger spatial scales.

Scaling Parameters of the Lewis-Kostiakov Water Infiltration Equation Across Soil Textural Classes and Extension to Rain Infiltration

A recent study showed that the pore-size distribution index (k) of the Brooks-Corey equation related and scaled cumulative infiltration (I) across eleven textural classes under different rainfall and initial conditions using normalization of the Green-Ampt equation or implicit empirical relations. The initial objectives herein were to (1) explore if more explicit, easy to use, and compact scaling could be achieved through relationships between the parameters of the empirical Lewis-Kostiakov (L-K) infiltration equation (I = cumulative infiltration = kt α ; t = time; a, k = constants) and or the effective saturated hydraulic conductivity (KS ) across eleven soil types for instantaneous incipient (zero-head) ponding cases; and (2) in the process, look for a more physical interpretation of the parameters and their dependence on initial soil water content. The Green-Ampt infiltration method in the Root Zone Water Quality Model (RZWQM) was used to generate simulated values for instantaneous zero-head infiltration at two initial pressure heads (-1500 and -100 kPa) in eleven homogeneous textural-class mean soils for 5 h, using the detailed Brooks-Corey hydraulic parameters for each soil. The two L-K parameters (a, k) were shown to have fairly strong explicit relationships with (r 2 = 0.78 to 0.88) and stronger relationships with K S (r 2 = 0.94 to 0.99) across the eleven textural classes. Additionally, a was essentially the same for the two initial pressure heads, and its value varied from 0.5 for clay soil to 0.58 for sand, indicating the dominance of sorptivity for clay and the increasing gravity effect for lighter textures, as expected from the theory. The intercept k varied with the pressure head condition but was related to the initial soil water deficit in the same way as sorptivity. Upper time limits for the L-K equation (t b ) to be applicable were also more strongly related to Ks (r 2 = 0.99) in all soils. A larger-time (beyond t b ) extension of the L-K equation proposed in the literature was also shown to be valid, thus making it more valuable. The L-K equation was then extended to non-instantaneous ponding infiltration for several rainfall intensities (I - Ip = k (t - tp)α'; Ip = I at incipient ponding time tp). The new parameters α' and k for each rainfall intensity were again found to be strongly related to K s or λ, and their variation with respect to initial pressure head was similar to that of a and k. This study provides a simple new method to quickly estimate the variation of infiltration with soil type on a landscape, scale up infiltration from small to large areas, and estimate effective average parameters for modeling large areas. The study also establishes a more physical basis for the L-K equation parameters and shows that it can be extended to large times and to infiltration of rainfalls, just like the Green-Ampt equation.

Soil and Water Assessment Tool Hydrologic and Water Quality Evaluation of Poultry Litter Application to Small-Scale Subwatersheds in Texas

The application of poultry litter to agricultural land has become a topic of interest for policy makers due to public concern about its effects on water quality. The Soil and Water Assessment Tool (SWAT) version 2005 is designed to assess nonpoint and point sources of pollution. In this study, six subwatersheds in Texas (HUC-8; 12070101) are used to evaluate the model's ability to simulate water quality at a small scale. Each of these subwatersheds randomly received poultry litter rates of 0.0 to 13.4 Mg ha-1. Monthly and daily data from 2002 were used for calibration purposes, while 2000, 2001, 2003, and 2004 were used for validation. The SCS runoff curve number for moisture condition II (CN2) and the soil evaporation compensation factor (ESCO) parameters were found to be more sensitive than the surface runoff lag time (SURLAG) and initial soil water content expressed as a fraction of field capacity (FFCB). The monthly and daily runoff model simulations for the six subwatersheds resulted in calibration Nash-Sutcliffe efficiency (NSE) values of 0.59 and 0.53 and validation NSE values 0.82 and 0.80, respectively. The monthly and daily R2 runoff values for the six subwatersheds resulted in calibration values of at least 0.60 and 0.53 and validation R2 values of 0.86 and 0.81, respectively. The observed trends included SWAT's overestimation of runoff in the dry periods and underestimation in the wet periods. The monthly NSE and R2 values for sediment and nutrient losses were generally above 0.4 and 0.5, respectively. Paired t-tests for the monthly manually adjusted parameter simulation of sediment, organic N and P, NO3-N, and soluble P for the 2000-2004 period losses showed that their respective SWAT means were not significantly different from the measured values (a = 0.05), except for NO3-N losses for the Y10 subwatershed (p-value 0.042). The control subwatershed's measured and simulated water quality results were significantly different (a = 0.05) from the treated subwatersheds, most likely due to the amount of inorganic N present. Almost all of the subwatersheds that had poultry litter applied resulted in higher sediment, organic N, organic P, and soluble P losses than the control subwatershed upon averaging the monthly validation values. High NO3-N losses may have been a function of poultry litter and commercial fertilizers being applied before a large rainfall event occurred. The subwatersheds that received smaller amounts of commercial fertilizer and/or poultry litter lost more sediment, organic N, and organic P than the subwatersheds that received the higher litter and/or fertilizer treatments. Overall, the SWAT simulated the hydrology and the water quality constituents at the subwatershed scale more adequately when all of the data were used to simulate the model, as evidenced by statistical measures.

Effects of decreasing soil water content on seminal lateral roots of young maize plants

AbstractSoil micropores that contain water at or below field capacity cannot be invaded by seminal or first‐order lateral roots of maize plants because their root diameters are larger than 10 μm. Hence, at soil‐water levels below field capacity plant roots must establish a new pore system by displacement of soil particles in order to access soil water. We investigated how decreasing soil water content (SWC) influences growth and morphology of the root system of young maize plants. Plants were grown in rhizotrons 40 cm wide, 50 cm high, and approximately 0.7 cm thick. Five SWC treatments were established by addition of increasing amounts of water to soil and thorough mixing before filling the rhizotrons. No water was added to treatments 1–4 throughout the experiment. Treatment 5 was watered frequently throughout the experiment to serve as a control. Seminal‐root length and SWC in soil layers 0–10, 10–20, 20–30, 30–40, and 40–50 cm were measured at intervals of 2–3 d on scanner images by image analysis. At 15 d after planting, for treatments 1–4 shoot dry weight and total root length were directly related to the amount of water added to the soil, and for treatments 4 and 5, total root length and shoot dry weights were similar. Length of seminal roots visible at the transparent surface of the rhizotron for all treatments was highest in the uppermost soil layer and decreased with distance from the soil surface. For all layers, seminal‐root elongation rate was at maximum above a SWC of 0.17 cm3 cm–3, corresponding to a matric potential of –30 kPa. With decreasing SWC, elongation rate decreased, and 20% of maximum seminal root elongation rate was observed below SWC of 0.05 cm3 cm–3. After destructive harvest for treatment 1–4, number of (root‐) tips per unit length of seminal root was found uninfluenced over the range of initial SWC from 0.10 to 0.26 cm3 cm–3. However, initial SWC close to the permanent wilting point strongly increased number of tips. Average root length of first‐order lateral (FOL) roots increased as initial SWC increased, and the highest length was found for the frequently watered treatment 5. The results of the study suggest that the ability to produce new FOL roots across a wide range of SWC may give maize an adaptive advantage, because FOL root growth can rapidly adapt to changing soil moisture conditions.

Sensitivity Analysis of Soil Hydraulic Properties on Subsurface Water Flow in Furrows

Knowledge of the sensitivity of various soil hydraulic properties is beneficial for model development and application purposes. It can lead to better estimated values, better understanding, and thus reduced uncertainty. In the present study, an extensive sensitivity analysis was performed to investigate the effects that various soil hydraulic properties have on subsurface water flow below furrows during two successive irrigation events to see which irrigation event was more sensitive and to analyze the effect of spatial variations in the initial soil water contents within the soil profile. Testing the sensitivity of the various soil hydraulic parameters in the van Genuchten-Mualem expression was carried out using the HYDRUS-2D model for two irrigation events 10 days apart. Results showed that the first irrigation event was clearly more sensitive than the second one. The latter event was mainly associated with the nonuniformity of the initial soil water contents within the soil profile. Pressure heads in t...

Estimation of Soil Hydraulic Properties from Numerical Inversion of Tension Disk Infiltrometer Data

Many applications involving variably saturated flow and transport require estimates of the unsaturated soil hydraulic properties. Numerical inversion of cumulative infiltration data during transient flow, complemented with initial or final soil water content data, is an increasingly popular approach for estimating the hydraulic curves. In this study, we compared Mualem–van Genuchten (MVG) soil hydraulic parameters obtained from direct laboratory and in situ unsaturated hydraulic conductivity measurements with estimates using numerical inversion of tension infiltration data of four coarse‐ to medium‐textured soils in Alentejo (Portugal). The laboratory methods used were suction tables, pressure plates, and the evaporation method as applied to undisturbed soil samples collected from the surface horizons of four different soil profiles. Field measurements were taken with a tension disk infiltrometer using consecutive supply pressure heads of −15, −6, −3, and 0 cm. Six MVG parameters (residual soil water content [θr], saturated soil water content [θs], empirical shape factorsα, η, and ℓ, and saturated hydraulic conductivity [Ks]) were estimated from the field data by numerical inversion using the HYDRUS‐2D software package, and compared with values estimated from the laboratory data. Macroporosity was also determined. The laboratory‐ and field‐measured water retention curves were found to agree closely for most experiments as reflected by relatively high values of the coefficient of determination, the modified coefficient of efficiency, and the modified index of agreement (always &gt;0.9949, 0.8412, and 0.8931, respectively). The unsaturated hydraulic conductivity curves were predicted less accurately, although good estimates of Ks were obtained.

The effect of initial soil water content and rainfall intensity on near-surface soil hydrologic conductivity: A laboratory investigation

This paper investigates the influence of rainfall intensity and initial soil water content on changes in the near-surface soil hydraulic conductivity. While numerous papers have examined surface sealing, this paper outlines two important innovations: the design and application of time-domain reflectometry probes with the ability to measure and record soil water content at very short time and length scales; the design and application of stainless steel tensiometers to measure soil water potential at the same, very short, time and length scales. These permit the Richards Equation to be applied and the near-surface hydraulic conductivity computed. An exponential curve was fitted to the temporal changes in hydraulic conductivity, as both a function of water potential or soil water content, with a high degree of fit. As rainfall intensity increased, the degree of scatter about the best-fit line increased, probably due to the ability of high intensity rainfall to disrupt the near-surface soil structure. While utilising deterministic models, such as the Richards Equation, may be appropriate at the column scale there are practical difficulties due to the very small length scale variability in each of the input variables. The experimental results detailed here indicate that even with data at very short time and length scales it is still not possible to utilize the Richards Equation, or at least, it may be difficult to transfer the laboratory results to the field due to the effect of the specific laboratory experimental conditions on the results.

Future directions for advanced evapotranspiration modeling: Assimilation of remote sensing data into crop simulation models and SVAT models

Soil-Vegetation-Atmosphere Transfer Models (SVAT) and Crop Simulation Models describe physical and physiological processes occurring in crop canopies. Remote sensing data may be used through assimilation procedures for constraining or driving SVAT and crop models. These models provide continuous simulation of processes such as evapotranspiration and, thus, direct means for interpolating evapotranspiration between remote sensing data acquisitions (which is not the case for classical evapotranspiration mapping methods). They also give access to variables other than evapotranspiration, such as soil moisture and crop production. We developed the coupling between crop, SVAT and radiative transfer models in order to implement assimilation procedures in various wavelength domains (solar, thermal and microwave). Such coupling makes it possible to transfer information from one model to another and then to use remote sensing information for retrieving model parameters which are not directly related to remote sensing data (such as soil initial water content, plant growth parameters, physical properties of soil and so on). Simple assimilation tests are presented to illustrate the main techniques that may be used for monitoring crop processes and evapotranspiration. An application to a small agricultural area is also performed showing the potential of such techniques for retrieving evapotranspiration and information on irrigation practices over wheat fields.

Improving the Infiltration of Water through Repellent Soils Using Synergistic Surfactant Blends Based on Alkyl Glucosides and Ethylene Oxide-Propylene Oxide Block Copolymers

Abstract Soil water repellency arises as a consequence of initial soil water content and the type of organic matter incorporated into or adsorbed onto the individual soil particles. Water repellent soils create significant agronomic and hydrologic challenges, retarding water infiltration into the soil profile and often rendering entire areas impervious to water penetration. These soils often prevent effective wetting of the rootzone during precipitation or irrigation and lead to increased runoff and leaching of pesticides, fertilizers, and other amendments. Agriculturalists have approached the soil water repellency problem through the use of wetting agent surfactant compositions. The degree of wetting efficacy among surfactant chemistries and formulations has varied significantly. The wetting characteristics of alkyl glucosides and ethylene oxide-propylene block copolymers, two nonionic surfactant classes, were evaluated in the context of improving the infiltration of water through water repellent soil profiles. A laboratory column test was used to measure the initial effectiveness of the wetting agent compositions on synthetically hydrophobized and naturally water repellent soils. A synergistically enhanced wetting rate occurred when the alkyl glucoside to block copolymer ratio was from approximately 6:1 to 0.5:1 by weight, ideally when the ratio was from approximately 4:1 to 0.7:1. Within these ratios, synergism was inversely proportional to the HLB and proportional to the molecular weight of the block copolymer used. This research is the first to demonstrate the promising effects of formulations comprising alkyl glucosides and EO/PO block copolymers on improving the distribution and dynamics of water in repellent soils.

A holistic generic integrated approach for irrigation, crop and field management: 1. The SALTMED model and its calibration using field data from Egypt and Syria

The SALTMED project has been carried out by establishing two field experiments in Egypt and Syria between 2000 and 2002. A salt tolerant variety of tomato ( Lycopersicon esculentum) known as Floradade has been tried in the field under different water management strategies from which the data were collected to develop and test the SALTMED model. The statistical design of the field experiment comprised two water application strategies: blended and cyclic, two methods of irrigations: trickle and furrow and six water salinity levels. Each treatment was replicated three times. The SALTMED model has been calibrated using the drip irrigation water treatment of 2002 in both Syria and Egypt. The calibration was primarily focused on yield prediction. The irrigation files contained field measurements of flow rate, duration of each irrigation and salinity of irrigation water. Plant parameters such as maximum plant height and rooting depth, length of each growth stage, planting date and harvesting date were based on field measurements and records. Crop coefficients such as K c, K cb, F c were based on ( Allen et al., 1998) which forms part of the SALTMED built-in crop database. Soil parameters such as the saturated and unsaturated hydraulic conductivity, were largely based on field measurements while the water retention curves were based on laboratory measurements. Initial soil water content and salinity were based on measurements either in laboratory or in the field. Fine tuning of some crop and soil parameters were carried out in order to obtain good calibration, i.e. close agreement between measured and simulated yield. These adjusted parameters successfully predicted the observed yield in both Egypt and Syria. Following successful calibration, the same set of crop parameters such as crop coefficient, K c, basal crop coefficient, K cb, fraction cover, F c, osmotic potential at which the potential water uptake is reduced by 50%, π50, maximum height and maximum rooting depth as well as the calibrated soil parameters were used for model testing in both Egypt and Syria for all seasons. The results are given in part two in this issue.

Mesoscale modelling of interactions between rainfall and the land surface in West Africa

Within the joint research project IMPETUS (An integrated approach to the efficient management of scarce water resources in West Africa), the effect of interactions between the Earth’s surface and the atmosphere on fresh water availability is investigated. Explorations are conducted for a river catchment in Benin by means of simulations with a non-hydrostatic mesoscale meteorological model. A combination of idealised ensemble simulations with a column version of the model and 3-D modelling of real precipitation events is employed to assess the sensitivity of precipitation to variations in the land surface. Simplified ensemble studies exhibit a dominant influence of initial soil water content and an enhanced dependence of precipitation on vegetation when soil water availability is reduced. For wet soils, the influence of parameters that determine the intensity of near-surface turbulence is dominant. 3-D modelling confirms that these relationships are useful to identify critical land use changes in realistic settings, but do not comprehensively account for the effect of heterogeneous land surface changes on regional precipitation. Instead, the interplay between surface properties, atmospheric dynamics and precipitation systems can generate intrinsic precipitation anomaly patterns that are incongruent with the imposed surface anomalies. Hence, assessments of land use change effects on precipitation for a specific region should be based on an integrated consideration of the interactions between surface processes, atmospheric forcing and precipitation systems. Based on these findings, possible effects of successive land degradation are investigated by sensitivity studies of land surface and rainfall system interaction for the Haute Vallée de l’Ouémé (HVO). In a first series of 3-D model simulations, a successive increase of the surface fraction with adverse conditions for the development of precipitation systems is performed. Within the scope of a second series a successive reduction of surface vegetation and soil water at randomly distributed areas that cover half of the simulation domain is carried out. Basically, a uniform decrease of average precipitation forced by changing conditions and a strong reduction of rainfall in some parts of the HVO are found. As a whole, the results strongly support the hypothesis of a growing risk of rainfall decrease as a result of land use changes.

Water and heat transport in hilly red soil of southern China: I. Experiment and analysis

Studies on coupled transfer of soil moisture and heat have been widely carried out for decades. However, little work has been done on red soils, widespread in southern China. The simultaneous transfer of soil moisture and heat depended on soil physical properties and the climate conditions. Red soil is heavy clay and high content of free iron and aluminum oxide. The climate conditions are characterized by the clear four seasons and the serious seasonal drought. The great air temperature differences annually and diurnally result in significant fluctuation in soil temperature in top layer. The closed and evaporating columns experiments with red soil were conducted to simulate the coupled transfer of soil water and heat under the overlaying and opening fields' conditions, and to analyze the effects of soil temperature gradient on the water transfer and the effects of initial soil water contents on the transfer of soil water and heat. The closed and evaporating columns were designed similarly with about 18 degrees C temperatures differences between the top and bottom boundary, except of the upper end closed or exposed to the air, respectively. Results showed that in the closed column, water moved towards the cold end driven by temperature gradient, while the transported water decreased with the increasing initial soil water content until the initial soil water content reached to field capacity equivalent, when almost no changes for the soil moisture profile. In the evaporating column, the net transport of soil water was simultaneously driven by evaporation and temperature gradients, and the drier soil was more influenced by temperature gradient than by evaporation. In drier soil, it took a longer time for the temperature to reach equilibrium, because of more net amount of transported water.