Fouad H. Jaber

Stability and accuracy of two-dimensional kinematic wave overland flow modeling

Abstract A two-dimensional finite element based overland flow model was developed and used to study the accuracy and stability of three numerical schemes and watershed parameter aggregation error. The conventional consistent finite element scheme results in oscillations for certain time step ranges. The lumped and the upwind finite element schemes are tested as alternatives to the consistent scheme. The upwind scheme did not improve on the stability or the accuracy of the solution, while the lumped scheme provided stable and accurate solutions for time steps twice the size of time steps needed for the consistent scheme. A new accuracy based dynamic time step estimate for the two-dimensional overland flow kinematic wave solution is developed for the lumped scheme. The newly developed dynamic time step estimates are functions of the mesh size, and time of concentration of the watershed hydrograph. Due to lack of analytical solutions, the time step was developed by comparing numerical solutions of various levels of discretization to a reference solution using a very fine mesh and a very small time step. The time step criteria were tested on a different set of problems and proved to be adequate for accurate and stable solutions. A sensitivity analysis for the watershed slope, Manning’s roughness coefficient and excess rainfall rate was conducted in order to test the effect of parameter aggregation on the stability and accuracy of the solution. The results of this analysis show that aggregation of the slope data resulted in the highest error. The roughness coefficient had a smaller effect on the solution while the rainfall intensity did not show any significant effect on the flow rate solution for the range of rainfall intensity used. This work pioneers the challenge of providing guidelines for accurate and stable numerical solutions of the two-dimensional kinematic wave equations for overland flow.

MIKE SHE: Model Use, Calibration, and Validation

MIKE SHE is a physically based, integrated water resources model that simulates both surface and subsurface water dynamics. These dynamics include interception, evapotranspiration, overland flow, channel flow, unsaturated flow, and saturated zone flow. The model can also simulate limited surface water quality processes using the advection dispersion equation and groundwater quality with a random-walk tracking method. This article is intended to describe the components of MIKE SHE and its calibration, validation, and applications. Given the deterministic, physically based characteristics of MIKE SHE and the large number of processes it simulates simultaneously, the number of parameters to calibrate can be large. A combination of user experience with the model, knowledge of the problem being solved, and an automatic calibration feature help in selecting the most sensitive parameters to calibrate. A case study describing the use of the MIKE SHE model, coupled with the hydraulic MIKE 11 model, to solve water flow in agricultural reservoirs in south Florida is presented. The problem required the use of the MIKE SHE model due to the high groundwater and overland flow interactions as well as the complex flow patterns inside the reservoir. The horizontal and vertical saturated hydraulic conductivities and the leakage coefficient were identified as the calibration parameters. The model was calibrated and evaluated for two reservoirs using two different time periods. Results showed matching between measured and simulated data for both reservoirs for the calibration and validation periods. The coupled MIKE SHE/MIKE 11 model has the capabilities to simulate complex hydrological processes and their interactions, which few other watershed models can match, and is suitable for current problems, such as the effects of climate change and land use change.

Impact of Organic Amendments on Groundwater Nitrogen Concentrations for Sandy and Calcareous Soils

Experiments were conducted on calcareous and sandy soils to investigate the effects of organic amendments for vegetable production on groundwater nitrogen (N) concentration in south Florida. The treatments consisted of applying yard and food residuals compost, biosolids compost, a cocompost of the municipal solid waste and biosolids, and inorganic fertilizer. Nitrate nitrogen (NO3-N), ammonium nitrogen (NH4-N), and total N concentrations were collected for a period of two years for both soils. Statistical analysis results revealed that for the three species tested, there were no significant differences among treatments. NO3-N concentrations for all treatments remained less than the maximum contamination level (10 mg/L). NO3-N transport to groundwater was higher in calcareous soil (mean=5.3 mg/L) than in sandy soil (mean=0.6 mg/L). NH4-N concentrations ranged from 0 to 13.6 mg/L throughout the experiment. Calcareous soil had lower NH4-N concentrations (mean=0.1 mg/L) than sandy soils (mean=0.7 mg/L). Total N ranged from 0.4 to 21.7 mg/L for all treatments for both soils reflecting high adsorption of dissolved organic N in both soils. Overall, results indicated that all the compost treatments were comparable to inorganic fertilizer with regard to N leaching and N concentrations in the groundwater while producing similar or higher yields.

Stability and accuracy of finite element schemes for the one-dimensional kinematic wave solution

Solving the kinematic wave equations for overland flow using the conventional consistent Galerkin finite element scheme is known to result in numerical oscillations due to the non-symmetric first spatial derivative terms in the kinematic wave equations. In this paper the lumped and the upwind finite element schemes are evaluated as alternatives to the consistent Galerkin finite element scheme. Stability analysis of the upwind scheme shows that the damping effect, that could reduce the oscillations, is small for the high Courant numbers encountered in overland flow problems. The upwind scheme, using upwind factors of 0.1 and 1.0, did not provide any improvement to the stability of the lumped and the consistent schemes. The lumped scheme considerably reduces oscillations without significant reduction in the overall solution accuracy. No analytical guidelines for time-step criteria that will insure stability and accuracy were provided by the stability analysis performed for the three schemes. Problem specific accuracy-based dynamic time-step criteria was developed and evaluated for the lumped scheme. These time-steps were found to be on average, double the size of the dynamic time-steps for the consistent scheme.

SIMULATING WATER DYNAMICS IN AGRICULTURAL STORMWATER IMPOUNDMENTS FOR IRRIGATION WATER SUPPLY

The hydrology of an impoundment in an agricultural grove in southern Florida was studied to assess its potential use as a water supply source. The MIKE SHE/MIKE 11 integrated hydrologic model was used to simulate the various hydrologic processes and their interactions. The model was calibrated and validated for water levels inside the impoundment and in a ditch outside the impoundment. Model evaluation results showed that the model could be used for assessing water retention alternatives with root mean square error (RMSE) values ranging from 0.03 to 0.25 m. Several structural and managerial alternatives were identified and evaluated to increase water retention volume and time. The alternatives include lining the impoundment, lining only the embankment and the inner distribution canal with 15 and 30 cm liner thicknesses, and pumping water regularly from the surrounding ditch to the impoundment. All alternatives were compared to the present condition. Lining the entire impoundment with clay provided ten weeks of additional irrigation from September to May. Lining the embankment and the inner canal of the impoundment provided up to four weeks of additional irrigation. Extending the regular pumping for a month after the wet season resulted in the reservoir being filled to full capacity during that month. The modeling study shows that clay-lining impoundments could provide an additional source of irrigation.

Groundwater Phosphorus and Trace Element Concentrations from Organically Amended Sandy And Calcareous Soils of Florida

The effects of organic amendments on vegetable crop production, phosphorus (P), and trace element (Zn, Cu, Mn, B, Cd, Pb, Ni) concentrations in groundwater were investigated on calcareous and sandy soils in south Florida. Treatments consisted of applying yard trash and food compost, biosolids compost, a cocompost of municipal solid waste and biosolids, and inorganic fertilizer. A randomized complete block design with four replications was used at both study sites. Total Kjeldahl phosphorus (TKP) and soluble reactive phosphorus (SRP) were periodically measured in grab samples collected for two years for both soils from wells above and below the spodic horizon at Ft. Pierce and from one depth at Homestead. Treatments were similar (P > 0.05) except on two sampling dates from the deep wells in the sandy soil at Ft. Pierce, one for SRP and one for TKP. Phosphorus concentrations for all treatments averaged < 1.2 mg SRP L−1 at Ft. Pierce and 0.04 mg SRP L−1 for the calcareous soil at Homestead. From solely a P consideration, organic P sources could be used to offset all or a portion of P required to satisfy vegetable crop nutrient requirements. This statement is consistent with crop yields at Ft. Pierce where yields from the three organic sources were equal to or exceeded inorganic fertilization yields. Micronutrient and trace element concentration responses to organic treatments were more pronounced at Ft. Pierce, perhaps due to soil chemical conditions including lower soil pH. The cocompost should be used cautiously to avoid contamination problems, especially when application rates are based on N fertilization as in this study. Due to the calcareous soil at Homestead, trace element concentrations in the groundwater were considerably lower than Ft. Pierce concentrations. However, the cocompost source was elevated for lead (Pb) (P < 0.05) compared with all other treatments, although higher concentrations were observed only on one sampling date. Results from this study indicated that, combined with the environmental benefits of recycling waste, the use of these organic amendments is a viable alternative to inorganic P fertilizers in the sandy and calcareous soils of peninsular Florida. The compost treatments were comparable to inorganic fertilizer with regard to P concentrations in the groundwater while producing similar or higher vegetable crop yields.

Effects of soil moisture sensor spacing and zone of influence on recharge calculations

Quantification of groundwater recharge is essential for water resources management. The water balance method is one of several methods for measuring event-based groundwater recharge. Soil moisture measurement data are essential for quantifying recharge using the water balance method. The spacing and zone of influence of soil moisture sensors used in a study are important factors for the accuracy of recharge calculations. A large lysimeter was used to measure the water balance components to quantify groundwater recharge in South Florida as a function of soil moisture and water table levels. Recharge was calculated using sensor spacings of 10 and 20 cm and zone of influence of 10 and 20 cm for seven major rainfall events. Recharge calculated with a sensor spacing of 20 cm had a relative root mean square error (RRMSE) of 33.1% as compared with the 10-cm spacing. Recharge calculated using a zone of influence of 20 cm had an RRMSE of 38.8% as compared with the 10-cm zone of influence. This error is attributed to soil moisture sensor discretization, which results in averaging the soil moisture in a soil layer containing the saturated and unsaturated zones interface, which leads to an overestimation of soil moisture in the unsaturated zone and an underestimation of soil moisture in the saturated zone. Vertical spacing of sensors larger than the sensors zone of influence also gives erroneous results as portions of the soil profile are not measured. For groundwater recharge calculations, selecting a sensor with suitable zone of influence and appropriate sensor spacing can have a significant effect on the accuracy of the calculations.

A new framework for modeling decentralized low impact developments using Soil and Water Assessment Tool

Assessing the performance of LID practices at a catchment scale is important in managing urban watersheds. Few modeling tools exist that are capable of explicitly representing the hydrological mechanisms of LIDs while considering the diverse land uses of urban watersheds. In this paper, we propose computational modules that simulate the hydrological processes of LIDs including green roof, rain garden, cistern, and porous pavement. The applicability of the modules was evaluated using plot scale experimental monitoring data. The effectiveness of LIDs was investigated in a highly urbanized watershed located in Austin, TX. Results indicate that the performance of LIDs is sensitive to LID configurations, application areas, and storm event characteristics, suggesting the need for studies on spatial optimization of LIDs and critical storm events to maximize the utility of LIDs. The LID modules offer a comprehensive modeling framework that explicitly simulates the water quantity processes of the LIDs considering landscape heterogeneity. We propose modules for the explicit simulation of low impact development practices.LID practices simulated include green roof, rain garden, cistern, and porous pavement.New modules are integrated with Soil and Water Assessment Tools sub-daily simulation.The integration enables to consider heterogeneous urban land uses in LID simulation.LID performance is responsive to storm event features as well as LID configuration.

Evapotranspiration losses for drip-irrigated watermelon in shallow water table and sandy soil conditions

Florida has been endowed with abundant water resources; however, with a population growth rate of nearly 23%, demand for water has increased to make water conservation one of the state’s priorities. Since agriculture accounts for 45% of total fresh water withdrawals, sound irrigation scheduling and efficient irrigation systems are key for optimum plant growth and conserving water quantity and quality. Estimating the crop evapotranspiration (ETc) by multiplying the crop coefficient (Kc) by the Penman-Monteith reference evapotranspiration (ETo) could help in more efficient agricultural water use. Accurate ETc can only be calculated using locally developed Kc values that take into account local production practices. A study was undertaken to develop crop coefficient (Kc) values for drip-irrigated watermelon, a common vegetable crop grown under plastic mulch in southwest Florida’s sandy soils. Four large drainage lysimeters (4.87m × 3.65m × 1.37m) were instrumented to measure inflow (irrigation and rainfall), outflow (drainage and runoff), and storage (soil moisture). Weather data were also collected. Using a bi-weekly time step, a Kc curve was developed using three seasons of data. The crop coefficient values for Florida were 0.57, 0.89, and 0.76 for March, April and May, respectively. The initial Kc is slightly higher than the values reported in FAO-56. This was mainly caused by the high soil moisture at the soil surface at the beginning of the season resulting from wetting of the soil for bed preparation. The middle and end Kc values were within the range FAO-56 values.

Bioretention and Permeable Pavement Performance in Clay Soil

Urbanization is altering the composition of landscapes nationwide, with urban areas characterized by a high proportion of impervious surfaces that adversely impact the water cycle of the region. The loss of infiltration of runoff into soil reduces ground water recharge. Increased surface runoff, velocity and pollution, all byproducts of rainfall on impervious surfaces, impede urban waterways tremendously. Increase in volume of runoff can lead to flooding, with receiving water bodies exhibiting stream bank erosion and channelization. Low impact development (LID) is considered to be a way to mitigate the adverse effects of increasing impervious cover, using decentralized measures to retain stormwater runoff on-site, and thereby seeking to mimic the natural pre-development hydrology of a site. Effectiveness of LID practices in various regions in the United States has been evaluated. However, modeling studies have suggested that the adaptability of LID designs to other regions is problematic, requiring modified solutions to be field tested in every location to confirm how they will perform. Therefore, there is still a great need to evaluate these practices in the field and to collect quantitative data on LID practices performance, especially in clay soils characterized by low infiltration. This project evaluates urban stormwater best management practices in a typical urban watershed in the Dallas Fort Worth area. The objectives were to design, construct and demonstrate the effectiveness of permeable pavements and bioretention area at the Texas A&M AgriLife Research and Extension Center in Dallas. Reduction in both volumes and pollutants concentration were recorded for all BMPS.