Guanhua Huang

Estimation of the van Genuchten soil water retention properties from soil textural data.

Abstract The van Genuchten (vG) function is often used to describe the soil water retention curve (SWRC) of unsaturated soils and fractured rock. The objective of this study was to develop a method to determine the vG model parameter m from the fractal dimension. We compared two approaches previously proposed by van Genuchten and Lenhard et al . for estimating m from the pore size distribution index of the Brooks and Corey (BC) model. In both approaches we used a relationship between the pore size distribution index of the BC model and the fractal dimension of the SWRC. A dataset containing 75 samples from the UNSODA unsaturated soil hydraulic database was used to evaluate the two approaches. The statistical parameters showed that the approach by Lenhard et al . provided better estimates of the parameter m . Another dataset containing 72 samples from the literature was used to validate Lenhards approach in which the SWRC fractal dimension was estimated from the clay content. The estimated SWRC of the second dataset was compared with those obtained with the Rosetta model using sand, silt, and clay contents. Root mean square error values of the proposed fractal approach and Rosetta were 0.081 and 0.136, respectively, indicating that the proposed fractal approach performed better than the Rosetta model.

Analytical solution of two-dimensional solute transport in an aquifer-aquitard system.

This study deals with two-dimensional solute transport in an aquifer-aquitard system by maintaining rigorous mass conservation at the aquifer-aquitard interface. Advection, longitudinal dispersion, and transverse vertical dispersion are considered in the aquifer. Vertical advection and diffusion are considered in the aquitards. The first-type and the third-type boundary conditions are considered in the aquifer. This study differs from the commonly used averaged approximation (AA) method that treats the mass flux between the aquifer and aquitard as an averaged volumetric source/sink term in the governing equation of transport in the aquifer. Analytical solutions of concentrations in the aquitards and aquifer and mass transported between the aquifer and upper or lower aquitard are obtained in the Laplace domain, and are subsequently inverted numerically to yield results in the real time domain (the Zhan method). The breakthrough curves (BTCs) and distribution profiles in the aquifer obtained in this study are drastically different from those obtained using the AA method. Comparison of the numerical simulation using the model MT3DMS and the Zhan method indicates that the numerical result differs from that of the Zhan method for an asymmetric case when aquitard advections are at the same direction. The AA method overestimates the mass transported into the upper aquitard when an upward advection exists in the upper aquitard. The mass transported between the aquifer and the aquitard is sensitive to the aquitard Peclet number, but less sensitive to the aquitard diffusion coefficient.

Modeling Soil Water Retention Curve with a Fractal Method

ABSTRACT Many empirical models have been developed to describe the soil water retention curve (SWRC). In this study, a fractal model for SWRC was derived with a specially constructed Menger sponge to describe the fractal scaling behavior of soil; relationships were established among the fractal dimension of SWRC, the fractal dimension of soil mass, and soil texture; and the model was used to estimate SWRC with the estimated results being compared to experimental data for verification. The derived fractal model was in a power-law form, similar to the Brooks-Corey and Campbell empirical functions. Experimental data of particle size distribution (PSD), texture, and soil water retention for 10 soils collected at different places in China were used to estimate the fractal dimension of SWRC and the mass fractal dimension. The fractal dimension of SWRC and the mass fractal dimension were linearly related. Also, both of the fractal dimensions were dependent on soil texture, i.e., clay and sand contents. Expressions were proposed to quantify the relationships. Based on the relationships, four methods were used to determine the fractal dimension of SWRC and the model was applied to estimate soil water content at a wide range of tension values. The estimated results compared well with the measured data having relative errors less than 10% for over 60% of the measurements. Thus, this model, estimating the fractal dimension using soil textural data, offered an alternative for predicting SWRC.

Effects of water deficits on growth, yield and water productivity of drip-irrigated onion (Allium cepa L.) in an arid region of Northwest China

Aiming at investigating an appropriate irrigation management strategy that could lead to increase onions yields and improve water productivity (WP), a two-year field experiment was conducted in the arid region of Northwest China with drip irrigation and plastic mulch. Eight treatments were considered: four with different levels of water stress throughout the crop season, and four where water stress was applied at the establishment, development, bulbification and ripening stages. The seasonal actual evapotranspiration (ETa), plant height, above-ground biomass, yield (total, high-quality and marketable quality yields) as well as related irrigation and total water productivity were determined. Plant heights, above-ground biomass and the referred yields have shown to be sensitive to water stress, particularly during the development and bulbification stages. Due to the importance of quality of horticultural products, the WP computed with the yields of high-quality bulbs revealed the most informative contrarily to the WP computed with the total yields. It could be concluded that water stress has to be avoided during the development and bulbification stages, and only small deficits are acceptable if applied throughout the crop season.

Quantitative estimation of climate change effects on potential evapotranspiration in Beijing during 1951–2010

Climate change is likely to affect hydrological cycle through precipitation, evapotranspiration, soil moisture etc. In the present study, an attempt has been made to study the climate change and the sensitivity of estimated evapotranspiration to each climatic variable for a semi-arid region of Beijing in North China using data set from 1951 to 2010. Penman-Monteith method was used to calculate reference crop evapotranspiration (ETo). Changes of ETo to each climatic variable was estimated using a sensitivity analysis method proposed in this study. Results show that in the past 60 years, mean temperature and vapor pressure deficit (VPD) were significantly increasing, relative humidity and sunshine hours were significantly decreasing, and wind speed greatly oscillated without a significant trend. Total precipitation was significantly decreasing in corn season (from June to September), but it was increasing in wheat season (from October to next May). The change rates of temperature, relative humidity, VPD, wind speed, annual total precipitation, sunshine hours and solar radiation were 0.42°C, 1.47%, 0.04 kPa, 0.05 m·s−1, 25.0 mm, 74.0 hours and 90.7 MJ·m−2 per decade, respectively. In the past 60 years, yearly ETo was increasing with a rate of 19.5 mm per decade, and total ETos in wheat and corn seasons were increasing with rates of 13.1 and 5.3 mm per decade, respectively. Sensitivity analysis showed that mean air temperature was the first key factor for ETo change in the past 60 years, causing an annual total ETo increase of 7.4%, followed by relative humidity (5.5%) and sunshine hours (−3.1%); the less sensitivity factors were wind speed (0.7%), minimum temperature (−0.3%) and maximum temperature (−0.2%). A greater reduction of total ETo (12.3%) in the past 60 years was found in wheat season, mainly because of mean temperature (8.6%) and relative humidity (5.4%), as compared to a reduction of 6.0% in ETo during corn season due to sunshine hours (−6.9%), relative humidity (4.7%) and temperature (4.5%). Increasing precipitation in the wheat season will improve crop growth, while decreasing precipitation and increasing ETo in the corn season induces a great pressure for local government and farmers to use water more efficiently by widely adopting water-saving technologies in the future.

Evaluation of Anomalous Solute Transport in a Large Heterogeneous Soil Column with Mobile-Immobile Model

This study uses the mobile-immobile model (MIM) and the traditional convection-dispersion equation (CDE) to analyze the observed breakthrough curves (BTCs) at different distances in a 1,250-cm-long saturated and highly heterogeneous soil column. It provides a simple method to determine the mobile water fraction independently as the ratio of effective porosity over total porosity of the packed soil materials. The effective porosity is calculated a priori as the ratio of measured flow rate and estimated pore-water velocity. It is found that there is a significant amount of immobile water in the soil column, resulting in the anomalous early breakthrough and tailing behaviors of the measured BTCs. Comparing to the CDE, the measured asymmetric BTCs at various scales can be better described by the MIM, especially their early arrival and long tailing parts. The degree of anomalous transport behavior in this large heterogeneous soil column is reduced with transport scale due to the increased mobile water fraction...

Experimental Investigation of Soil Evaporation and Evapotranspiration of Winter Wheat Under Sprinkler Irrigation

Abstract Sprinkler irrigation is one of the typical irrigation technologies used for the winter wheat-summer maize double cropping system in the North China Plain. To evaluate the evapotranspiration (ET) of winter wheat under sprinkler irrigation in Beijing area, field experiments were conducted in growing seasons through 2005–2008, in the experimental station located in Tongzhou County, Beijing, China, with different irrigation depths. Results indicated that a relatively large variation of soil water content occurred within 0–40 cm soil layer. The seasonal ET of winter wheat generally increased with increasing irrigation amount, while the seasonal usage of soil water had a negative relationship with irrigation amount. Soil evaporation (E s ) was about 25% of winter wheat ET during the period from reviving to maturity. E s increased while E s /ET decreased with increasing irrigation amount. Sprinkler irrigation scheduling with relatively large irrigation quota and low irrigation frequency can reduce E s and promote the irrigation water use efficiency.

Non-Darcian Flow Toward a Finite-Diameter Vertical Well in a Conflned Aquifer

Abstract Non-Darcian radial flow toward a flnite-diameter, fully penetrating well in a conflned aquifer was analyzed on the basis of the Izbash equation with consideration of the wellbore storage efiect. We derived semi-analytical solutions of drawdown by using the Boltzmann transform, and obtained approximate analytical solutions of the drawdown at early and late times. MATLAB programs were developed to facilitate computation of the semi-analytical solutions. The turbulence factor v which was directly related to the pumping rate appeared to have negligible influence upon the wellbore well function at early times, but imposed signiflcant infl;uence at intermediate and late times. However, the turbulence factor v imposed non-negligible influence upon the aquifer well function during the entire pumping period, provided that the observation point was not sufficiently close to the wellbore. Sensitivity analysis indicated that the power index n in the Izbash equation had less influence on the type curves at the face of the pumping wellbore, but had much greater influence upon the well function in the aquifer. As the n values increased, the drawdown in the aquifer decreased at early times and increased at late times. The Boltzmann transformation could only be used in an approximate sense for radial non-Darcian flow problems. This approximation would provide accurate solutions at early times, and introduce small but consistent discrepancies at intermediate and late times for the wellbore well function.

Global sensitivity analysis and calibration of parameters for a physically-based agro-hydrological model

Efficient parameter identification is an important issue for mechanistic agro-hydrological models with a complex and nonlinear property. In this study, we presented an efficient global methodology of sensitivity analysis and parameter estimation for a physically-based agro-hydrological model (SWAP-EPIC). The LH-OAT based module and the modified-MGA based module were developed for parameter sensitivity analysis and inverse estimation, respectively. In addition, a new solute transport module with numerically stable schemes was developed for ensuring stability of SWAP-EPIC. This global method was tested and validated with a two-year dataset in a wheat growing field. Fourteen parameters out of the forty-nine total input parameters were identified as the sensitive parameters. These parameters were first inversely calibrated by using a numerical case, and then the inverse calibration was performed for the real field experimental case. Our research indicates that the proposed global method performs successfully to find and constrain the highly sensitive parameters efficiently that can facilitate application of the SWAP-EPIC model. Global sensitivity analysis method was developed for SWAP-EPIC based on LH-OAT.A modified-MGA based module was developed for global inverse parameter estimation.A new solute transport module was developed with more numerical stability.Global method was well tested using numerical and real field experimental cases.

Modeling contribution of shallow groundwater to evapotranspiration and yield of maize in an arid area

Capillary rise from shallow groundwater can decrease the need for irrigation water. However, simple techniques do not exist to quantify the contribution of capillary flux to crop water use. In this study we develop the Agricultural Water Productivity Model for Shallow Groundwater (AWPM-SG) for calculating capillary fluxes from shallow groundwater using readily available data. The model combines an analytical solution of upward flux from groundwater with the EPIC crop growth model. AWPM-SG was calibrated and validated with 2-year lysimetric experiment with maize. Predicted soil moisture, groundwater depth and leaf area index agreed with the observations. To investigate the response of model, various scenarios were run in which the irrigation amount and groundwater depth were varied. Simulations shows that at groundwater depth of 1 m capillary upward supplied 41% of the evapotranspiration. This reduced to 6% at groundwater depth of 2 m. The yield per unit water consumed (water productivity) was nearly constant for 2.3 kg/m3. The yield per unit water applied (irrigation water productivity) increased with decreasing irrigation water because capillary rise made up in part for the lack of irrigation water. Consequently, using AWPM-SG in irrigation scheduling will be beneficial to save more water in areas with shallow groundwater.