Comparison of Field Data and Water-Balance Predictions for a Capillary Barrier Cover

Predictions of surface runoff (R), evapotranspiration (ET), soil–water storage (S), and percolation obtained using three numerical codes (LEACHM, HYDRUS, and UNSAT-H) employed to simulate the hydrology of water-balance covers are compared to measured water-balance data from a lysimeter used to monitor a capillary barrier cover profile in a subhumid climate. All of the codes captured the seasonal variations in water-balance quantities observed in the field. LEACHM and HYDRUS predicted total R during the monitoring period with reasonable accuracy (within 18?mm using general mean parameters), but the timing of predicted and observed R events was different. In contrast, UNSAT-H consistently overpredicted R by at least 239?mm. Evapotranspiration was predicted reliably (within 60?mm) with all three codes when data from the first year were excluded. However, all three codes overpredicted ET in late winter and early spring, when snowmelt was occurring and S was accumulating in the field. Consequently, S generally was underpredicted by all three codes. Predicted and measured percolation were in good agreement (within 1?mm/year), except during the first year. Results of the comparison indicate that cover modelers should scrutinize runoff predictions for reasonableness and carefully account for snow accumulation, snow melt, and ET during snow cover.     

Field Data and Water-Balance Predictions for a Monolithic Cover in a Semiarid Climate

Water-balance predictions made using four codes (UNSAT-H, VADOSE/W, HYDRUS, and LEACHM) are compared with water-balance data from a test section located in a semiarid climate simulating a monolithic water-balance cover. The accuracy of the runoff prediction (underprediction or overprediction) was found to affect the accuracy of all other water-balance quantities. Runoff was predicted more accurately when precipitation was applied uniformly throughout the day, the surface layer was assigned higher saturated hydraulic conductivity, or when Brooks-Corey functions were used to describe the hydraulic properties of the cover soils. However, no definitive or universal recommendation could be identified that would provide reasonable assurance that runoff mechanisms are properly simulated and runoff predictions are accurate. Evapotranspiration and soil-water storage were predicted reasonably well (within ≈ 25?mm/yr) when runoff was predicted accurately, general mean hydraulic properties were used as input, and the vegetation followed a consistent seasonal transpiration cycle. However, percolation was consistently underpredicted (>3?mm total) even when evapotranspiration and soil-water storage were predicted reliably. Better agreement between measured and predicted percolation (or a more conservative prediction) was obtained using mean properties for the soil-water characteristic curve and increasing the saturated hydraulic conductivity of the cover soils by a factor between 5 and 10. Evapotranspiration and soil-water storage were predicted poorly at the end of the monitoring period by all of the codes due to a change in the evapotranspiration pattern that was not captured by the models. The inability to capture such changes is a weakness in current modeling approaches that needs further study.     

Capillary Barriers: Design Variables and Water Balance

Water balance simulations were conducted with the unsaturated flow model UNSAT-H to assess how layer thicknesses, unsaturated hydraulic properties, and climate affect the performance of capillary barriers. Simulations were conducted for four locations in semiarid and arid climates. Hydraulic properties of four finer-grained and two coarser-grained soils were selected to study how saturated and unsaturated hydraulic properties affect the water balance. Results of the simulations indicate that thickness and hydraulic properties of the surface layer significantly affect the water balance of capillary barriers. As expected, increasing the thickness or reducing the saturated hydraulic conductivity of the finer-grained surface layer reduces percolation. Unsaturated hydraulic properties of the coarser layer also affect the water balance, including the storage capacity of the surface layer as well as the onset and amount of percolation from the cover. Thickness of the coarser layer has a much smaller impact on the water balance. Climate also affects the water balance. Greater soil water storage capacity is required at sites where the season with more frequent and less intense precipitation does not coincide with the season having highest evapotranspiration.     

New Weighing Method to Measure Shoot Water Interception

The need exists to develop a method that can quantitatively measure water interception from plant shoots. This paper describes a new method for measuring canopy water interception. Corn (Zea Mays L.) was grown in 13?L buckets containing Valentine fine sand (Mixed, mesic Typic Ustipsamment) under a climate-controlled growth chamber. Plants were taken out of the growth chamber for 2–3?h periods for measurements of shoot water interception in a hydraulic laboratory equipped with an Accupulse system suspended from the ceiling that was used to wet the corn shoots at growth stages V7–V13. A lightweight, movable framework was placed around a balance, and buckets containing corn plants were placed on the scale one container at a time. Water was applied until all shoot surfaces were wet and runoff from the leaves and stalk surfaces could be observed. The weighing method for shoot water interception was tested by using the balance to instantaneously measure shoot water interception during application of water and after plant surface runoff ceased. The balance, bucket, and soil surface were covered with plastic to protect them from water, so only the shoots were wet. Interception by the shoot of corn ranged from 31?to?47?mL?shoot?1. These values were much smaller than previous values reported in the literature. The average coefficient of variation was 9.2% for two studies, which was much lower than previously accepted methods. This study suggests that the weighing method for shoot water interception can be used to quantitatively and more accurately measure water intercepted by corn shoots. The weighing intercepted method values presented in this paper are low and suggest that previous interception methods overestimated the interception values.     

River Hydrograph Retransmission Functions of Irrigated Valley Surface Water–Groundwater Interactions

Storage and release functions of western U.S. traditional river valley irrigation systems may counteract early and rapid spring river runoff associated with climate variation. Along the Rio Grande in northern New Mexico, we instrumented a 20-km-long irrigated valley to measure water balance components from 2005 to 2007. Hydrologic processes of the system were incorporated into a system dynamics model to test scenarios of changed water use. Of river water diverted into an earthen irrigation canal system, some was consumed by crop evapotranspiration (7.4%), the rest returned to the river as surface return flow (59.3%) and shallow groundwater return flow that originated as seepage from canals (12.1%) and fields (21.2%). The modeled simulations showed that the coupled surface water irrigation system and shallow aquifer act together to store water underground and then release it to the river, effectively retransmitting river flow until later in the year. Water use conversion to nonirrigation purposes and reduced seepage from canals and fields will likely result in higher spring runoff and lower fall and winter river flow.     

Plant Water Use and Crop Curves for Hybrid Poplars

Four years of weekly soil water data measured by neutron probe were analyzed to determine average daily, monthly, and seasonal drip-irrigated hybrid poplar water use. The plantation studied is located near Boardman, Oregon, on the Columbia River Plateau. Irrigation application data, weekly rainfall, and changes in soil water content permitted the construction of a soil water balance model to calculate weekly hybrid poplar water use. Drainage was estimated by calculating potential soil water drainage from the lower soil profile. Sites with the potential for significant drainage were removed from the analysis, so that all sites used in the analysis could be assumed to be at steady state. Crop coefficients were calculated using reference evapotranspiration estimates obtained from a nearby AGRIMET weather station. Crop curves were estimated using a fit-by-hand method similar to that outlined by the United Nations Food and Agriculture Organization. Plant water use estimates and crop curves are presented for one-, two- and three-year-old hybrid poplars. Water use estimates represent upper bound estimates relative to the accuracy of the measurements made.     

Water Management of Irrigated-Drained Fields in the Jordan Valley South of Lake Kinneret

The Jordan Valley is one of the primary regions for growing winter crops of fruit and vegetables in Israel and Jordan. Control of water management in these fields is obtained by solid-set irrigation systems and subsurface drainage. Detailed field observations were conducted at a location near the Jordan River, south of Lake Kinneret. Water table heights were measured by approximately 100?piezometers. An exiting wide spacing (160?m) subsurface drainage system was monitored and the total drainage discharge from this regional drainage system to Lake Kinneret was measured. Rainfall, irrigation, and evapotranspiration rates were measured and overall hydrological balance was conducted. The old irrigation method in the region was border irrigation with very high leaching fraction and poor irrigation efficiency. During the 1970s the irrigation method was changed to computer operated drip irrigation. The leaching fraction was reduced and irrigation efficiency increased. Reduction of the total drainage discharge to Lake Kinneret by a factor of about 10 was observed. Water table rise under hand moving sprinkler and soil-set drip irrigation methods were measured and compared for assessment of salinization of the root zone by upward movement of groundwater. The result indicates the strong effect of irrigation time interval on the extent of these rises. The effect of irrigation mode on the extent of water table rises was measured at the field by comparing that under hand moving sprinkler irrigation to that under water solid set drip method. This effect depends, among other variables, on the irrigation time interval, a fact which complicates prediction of water table rise under different irrigation practices. These field results support previous theoretical analysis by the writers and highlighted the interrelationship between irrigation practice and drainage design. The effect of water table drawdown towards the Jordan River was monitored and found to be about 4.6%. The strong influence of the Jordan River on water table height at the drained field is magnified by the existence of sandy layers in the soil profile. This observed gradient may be used for the estimation of lateral seepage flow from the irrigated agricultural field towards the adjacent Jordan River. This study provides a useful source of data for future studies in similar situations.     

Performance Evaluation and Uncertainty Measurement in Irrigation Scheduling Soil Water-Balance Approach

The present work aims at approaching the study of the performance and uncertainty associated with an irrigation scheduling method based on a soil-water balance. On a daily time step, a water-balance-based irrigation scheduling model has been developed. A Monte Carlo simulation of the irrigation scheduling model is developed using a series of actual daily weather data of evapotranspiration and precipitation and bootstrapping stochastic technique to resampling them. Performance evaluation measurements and their uncertainty are studied by means of several parameters: reliability, resiliency, vulnerability, total irrigation water allocation, total water loosed by deep percolation, and actual evapotranspiration/potential evapotranspiration rate along the growing season. The behaviors of 12 different types of soils (between coarse-textured soils and fine-textured soils) are compared using pedotransfer functions. Total available water (TAW) is the most important hydraulic property of the soil as far as irrigation scheduling performance is concerned. The statistical relationship between evaluation performance measures and TAW has been calculated. Soils with high values of TAW perform better. Rooting depth (Zr) and fraction of TAW that can be depleted from the root zone before moisture stress (p) are two variables that directly affect the TAW. It has been studied how evaluation performance measurements change when Zr and p change too. High values of Zr and p perform better too.     

Irrigation Performance using Hydrological and Remote Sensing Modeling

Development of water saving measures requires a thorough understanding of the water balance. Irrigation performance and water accounting are useful tools to assess water use and related productivity. Remote sensing and a hydrological model were applied to an irrigation project in western Turkey to estimate the water balance to support water use and productivity analyses. Remote sensing techniques can produce high spatial coverage of important terms in the water balance for large areas, but at the cost of a rather sparse temporal resolution. Hydrological models can produce all the terms of the water balance at a high temporal, but low spatial resolution. Actual evapotranspiration for an irrigated area in western Turkey was calculated using the surface energy balance algorithm for land (SEBAL) remote sensing land algorithm for two Landsat images. The hydrological model soil-water-atmosphere-plant (SWAP) was setup to simulate the water balance for the same area, assuming a certain distribution in soil properties, planting dates and irrigation practices. A comparison between evapotranspiration determined from SEBAL and from SWAP was made and differences were minimized by adapting the distribution in planting date and irrigation practice. The optimized input data for SWAP were used to simulate all terms of the accumulated water balance for the entire irrigation project, and subsequently used to derive the irrigation performance indicators. The innovative methodology presented is attractive as it diminishes the need of field data and combines the strong points of remotely sensed techniques and hydrological models.     

Estimating Reference Evapotranspiration with Minimum Data in Florida

Reference evapotranspiration estimation methods that require minimal data are necessary when climatic data sets are incomplete, inaccurate, or unavailable. This study was conducted to evaluate temperature-based reference evapotranspiration methods in Florida. Using reference evapotranspiration estimates using satellite-derived radiation as the standard for comparison, the “reduced-set” Penman-Monteith, Hargreaves, and Turc equations were evaluated using monthly temperature data from 72 weather stations in Florida. The reduced-set Penman-Monteith equation requires maximum and minimum temperature only and uses recommended methods to estimate radiation, humidity, and wind speed. The reduced-set Penman-Monteith and Hargreaves equations were found to overestimate reference evapotranspiration while the Turc equation neither overestimated nor underestimated. The reduced-set Penman-Monteith equation showed greatest error in coastal stations while the Hargreaves equation showed greatest error at inland and island locations. In the absence of regionally calibrated methods the Turc equation is recommended for estimating reference evapotranspiration using measured maximum and minimum temperature and estimated radiation in Florida.     

Storage Volume and Overflow Risk for Infiltration Basin Design

This study presents a risk-based approach for the design of infiltration basins. The design parameters include basin storage volume, drain time, and overflow risk. At a basin site, the storm-water detention storage volume is determined by design runoff capture volume, tributary watershed area, and runoff coefficient. The basin geometry is dictated by the water volume balance between the surface storage volume in the basin and the subsurface storage capacity in soil pores. The drain time at a basin site is found to be a function of initial soil water content, soil porosity, infiltration rate, and distance to the ground-water table. After knowing the basin geometry and size, the basin's performance can be evaluated by the overflow risk analysis using the local average event rainfall depth and interevent time. In practice, a sensitivity test on overflow risk can be conducted with a range of basin storage volumes. The risk-based approach presented in this study provides an algorithm to calculate the long-term runoff capture percentage for a basin size. The diminishing return on runoff capture percentage can serve as a basis to select the proper basin storage volume at the site.     

Influence of Stress State on Soil-Water Characteristics and Slope Stability

A soil-water characteristic curve defines the relationship between the soil (matric) suction and either the water content or the degree of saturation. Physically, this soil-water characteristic is a measure of the water storage capacity of the soil for a given soil suction. Conventionally, the soil-water characteristic curves (SWCCs) are determined in the laboratory using a pressure plate apparatus in which vertical or confining stress cannot be applied. For investigating the influence on the stress state on the soil-water characteristics, a new stress controllable pressure plate apparatus has been developed. Effects of K0 stress conditions on the SWCCs of an “undisturbed” volcanic soil in Hong Kong are determined and illustrated. The net normal stresses considered in the apparatus are 40 and 80 kPa, which are appropriate for many slope failures in Hong Kong. Experimental results show that the soil-water characteristic of the soil specimens is strongly dependent on the confining stress. Numerical analyses of transient seepage in unsaturated soil slopes using the measured stress-dependent soil-water characteristic curves predict that the distributions of pore-water pressure can be significantly different from those predicted by the analyses using the conventional drying SWCC. For the cut slope and the rainfall considered, the former analyses predicted a considerably lower factor of safety than that by the latter analyses. These results suggest that wetting stress-dependent soil-water characteristic curves should be considered for better and safer assessment of slope instability.     

SEBAL Model with Remotely Sensed Data to Improve Water-Resources Management under Actual Field Conditions

Water management emphasis tends to shift from supply augmentation to limiting water consumption. Spatio-temporal information on actual evapotranspiration (ET) helps users to better understand evaporative depletion and to establish links between land use, water allocation, and water use. Satellite-based measurements, used in association with energy balance models, can provide the spatial distribution of ET for these linkages. This paper describes the major principles of the Surface Energy Balance Algorithm for Land (SEBAL) and summarizes its accuracy under several climatic conditions at both field and catchment scales. For a range of soil wetness and plant community conditions, the typical accuracy at field scale is 85% for 1?day and it increases to 95% on a seasonal basis. The accuracy of annual ET of large watersheds was found to be 96% on average. SEBAL has been applied in more than 30 countries worldwide, and the 26 research studies that were conducted over the past 10?years are now gradually being replaced by application studies (17 studies finished). A short case study in the Yakima River basin (Washington State) is presented as new material to demonstrate how ET from remote sensing can be used for evaluating water conservation projects.     

Solute Transport through Unsaturated Soil due to Evaporation

A new method for predicting upward solute movement in unsaturated sand soil due to evaporation is developed and tested. Laboratory experiments were conducted in an unsaturated uniformly packed sand column with a cross section of 1.20 × 0.50 m2 and a constant shallow ground-water table. Evaporation was measured by a new ventilated chamber system. Solute movement from the ground water upward was monitored. Water and solute movement could be accurately reproduced by numerically solving Richards' equation and the convection-dispersion equation in one-dimension. The experimentally measured dispersivity for the unsaturated homogeneous sand agreed closely with the values which are available in the literature. This paper offers a new approach for investigating dispersion phenomena in unsaturated porous media exposed to evaporation.     

Statistical Analysis of Reference Evapotranspiration on the Tibetan Plateau

Net radiation is an important and site-specific component to determine reference evapotranspiration (ET0). The empirical Angstrom coefficients for radiation estimation in the FAO56 Penman–Monteith (PM) equation were calibrated using observed daily solar radiation and actual sunshine duration on the Tibetan Plateau. The calibrated Angstom coefficients included annual coefficients for nine meteorological stations separately and aggregation of nine stations. The calibrated annual coefficients for each station separately were applied to estimate net radiation and further employed to estimate ET0 using the PM and the Priestley–Taylor (PT) equations on the Tibetan Plateau. Moreover, the Hargreaves (Harg) equation that requires only air temperature was also applied to estimate ET0. Comparisons of three methods were conducted and the results showed that the PT method overestimated daily ET0 with respect to PM–ET0 and the Harg method underestimated it at all meteorological stations. The PT method was more suitable for the study area in the absence of the parameters necessary for the calculation of PM–ET0. The Harg equation provides ET0 estimates when only air temperature is available and local calibration in the study can be applied on the Tibetan Plateau.     

Probabilistic Approach for Design and Hydrologic Performance Assessment of Reconstructed Watersheds

The oil sands mining industry in Canada has made a commitment to reclaim mining areas to an equivalent capability to that which existed prior to mining. An essential requirement in the design of reclamation covers to meet this objective is that all covers must have a sufficient available water holding capacity (AWHC) in order to supply sufficient moisture for vegetation over the summer moisture deficit typical in the region. AWHC is currently based on static evaluations of wilting point and field capacity under a constant annual evapotranspiration demand. This paper presents an alternative probabilistic approach by which the hydrologic performance of these reclamation soil covers can be assessed. A field-calibrated water balance model is used along with the available historical meteorological record to estimate the maximum soil moisture deficit that a soil cover is able to sustain over the growing season. Frequency curves of the maximum annual moisture deficit are used to assess the probability that a cover is able to provide any particular threshold of moisture demand. The method also allows for a quantification of the predictive uncertainty of the model. The predictive uncertainty is used as a margin of safety to estimate a design value of moisture deficit for various alternative cover designs. This paper recommends procedures for a frequency-based assessment and design of reclamation soil covers in the oil sands industry. This method takes into account climatic variability as well as parameter uncertainty in estimating the soil moisture deficit.     

Runoff Characteristics for Construction Site Erosion Control Practices

Controlling soil erosion during and after construction is a major concern due to the impacts of sediment on stream water quality, and many studies have focused on the effectiveness of erosion control best management practices (BMPs) to prevent erosion. However, their ability to reduce runoff volume and peak discharge is not commonly studied or integrated into storm water designs due to lack of data and design guidelines. This study investigated runoff characteristics (total runoff, peak flow rate, curve number, and rational method runoff coefficient) from bare compacted soil conditions with and without erosion control BMPs, with an emphasis on compost erosion control blankets (CECBs), at three different slope (2H:1V, 3H:1V, and 4H:1V). Experiments were performed in the San Diego State University, Soil Erosion Research Laboratory on a 3-m by 10-m indoor titling soil bed using simulated rainfall based on conditions specified in ASTM D-6459. Eleven erosion control BMPs were evaluated at a slope of 2H:1V, three at 3H:1V, and three at 4H:1V. The variations in slope were used to assess the effects of slope and CECB thickness on runoff. The results from this study provide new insight regarding the runoff characteristics from bare soil and erosion control BMPs that can be used to improve construction-site storm water design. The results show that 2.5- and 5.0-cm-thick CECBs on top of netting or an excelsior fiber blanket provided a significant reduction in runoff relative to the bare soil and the other BMPs (e.g., 1.3-cm CECBs, other blankets) due to water storage within the CECB, the mass of the CECB providing a strong bond between the soil surface and the bottom of the blanket reducing the potential for flowing water from coming in contact with the soil surface, and the netting/blanket under the CECB providing additional friction that helps keep the CECB from sliding down slope. The results show that slope impacts on runoff are minimal but that as CECB thickness increases runoff was reduced due to the added storage within the blanket. The results from this study can be used to aid in the selection of CECB thickness and to assess the effects of CECBs on runoff for more efficient cost effective storm water designs.     

Suction-Monitored Direct Shear Testing of Residual Soils from Landslide-Prone Areas

The apparent cohesion due to soil suction plays an important role in maintaining the stability of steep unsaturated soil slopes with deep ground water table. In this paper, a modified direct shear box is used to determine the relationships between the value of this additional cohesion and the associated soil suction. The apparatus incorporates a miniature tensiometer which allows for the simple and direct measurement of suction during shearing. The soil-water characteristic curves and shearing behavior of intact residual soils, being low-to-medium plasticity silts, as well as silty sand, taken from four landslide-prone areas in Thailand, have been investigated. The relatively low air-entry suctions (0–7 kPa) and bimodality of the soil-water characteristic curves gives an indication of the structured pore size distribution of the materials tested. Samples with higher suction tend to display stronger bonding at particle contacts and thus are more brittle. The shear strength is found to increase nonlinearly with suction, though linearization can be reasonably assumed for suction below around 30 kPa. Prediction of shear strength based on soil-water characteristic curves agrees better with ultimate than peak values. A simple equation is proposed for the minimum ultimate strength that can be expected in an unsaturated residual soil with a suction lower than about 30 kPa.     

1. Field and Laboratory Studies of Acid Sulfate Soils

Drains are introduced into acid sulfate (AS) soils grown to sugar cane to prevent waterlogging and drain runoff water. Drains have the potential to promote deleterious reactions and facilitate the transport of the resulting reaction products into the ecosystem. A field experiment was carried out to investigate the hydrology of an AS soil field and monitor the quality of its drainage water. Results have shown that in such low-conductivity soils, a steep water-table draw-down occurs close to the drain. Farther away from the drain, water-table dynamics are predominantly driven by evapotranspiration. The concentration of sulfate ions in the drainage water showed a steep decline during infiltration followed by a moderate surge during drainage. A laboratory leaching column experiment has revealed an increasing sulfate concentration away from the drain. The column experiment confirmed earlier findings of Rassam and Cook, who conducted hypothetical numerical simulations and showed that solutes from low-conductivity AS soils are mainly leached from soils located close to the drain.