Journal of Irrigation and Drainage Engineering

The objective of this study was to test an artificial neural network (ANN) for estimating the reference evapotranspiration (ETo) as a function of the maximum and minimum air temperatures in the Campos dos Goytacazes county, State of Rio de Janeiro. The data used in the network training were obtained from a historical series (September 1996 to August 2002) of daily climatic data collected in Campos dos Goytacazes county. When testing the artificial neural network, two historical series were used (September 2002 to August 2003) relative to Campos dos Goytacazes, and Viçosa, State of Minas Gerais. The ANNs (multilayer perceptron type) were trained to estimate ETo as a function of the maximum and minimum air temperatures, extraterrestrial radiation, and the daylight hours; and the last two were previously calculated as a function of either the local latitude or the Julian date. According to the results obtained in this ANN testing phase, it is concluded that when taking into account just the maximum and minimum air temperatures, it is possible to estimate ETo in Campos dos Goytacazes.

Cover cropping practices are being researched to reduce artificial subsurface drainage nitrate-nitrogen (nitrate-N) losses from agricultural lands in the upper Mississippi watershed. A soil-plant-atmosphere simulation model, RyeGro, was developed to quantify the influence of a winter cereal rye cover crop on nitrate-N losses given climatic variability in the region. This paper describes the hydrology and nitrogen cycle submodels of RyeGro, which was developed with a low level of complexity and conceptualizes the soil profile as three soil layers. The model was calibrated with data from a three-year rye cover crop field study conducted at Lamberton, Minnesota, and validated with data from a previous study. During model calibration, field subsurface drainage nitrate-N loadings were predicted within 0.2, 0, and $1.6\mathrm{kg}\mathrm{N}{\mathrm{ha}}^{-1}$ (1, 0, and $-3\%$ ) of measured loadings for the corn-soybean treatment and within 1.2, 0, and $1.6\mathrm{kg}\mathrm{N}{\mathrm{ha}}^{-1}$ (11, 0, and 3%) of measured loadings for the corn-rye-soybean treatment. The model validation showed nitrate-N loading differences of 7 and $1\mathrm{kg}\mathrm{N}{\mathrm{ha}}^{-1}$ ( $-22$ and 4%) for the two years tested.

In this paper, we compare the effect of two modes of irrigation on water transport through an unsaturated porous medium. The two modes studied are (1) periodic and (2) continuous irrigation. The former process consists of two stages, i.e., imbibition and drainage. The entire volume of water is assumed to be added during the imbibition stage at a constant rate. The time during which imbibition occurs is taken as a fraction of the total time period. In the drainage stage, no fresh water is added and the water inside the soil redistributes itself and drains from the soil. In the continuous mode, water is added at a constant rate during the whole time period. To ensure a fair comparison, the rate of water addition in the continuous mode is kept the same as the average rate of water addition over a time period in the periodic mode that includes the drainage and imbibition steps. The recharge is calculated as the volume of water drained from the bottom of the soil during a time period. The transport of water in the unsaturated zone is studied in the presence of water uptake by plant roots. The two modes of operation were simulated using a mass conservative algorithm based on a modification of Picard’s iterative scheme. Predictions in the periodic mode were performed using direct simulation and the results obtained were compared with an algorithm based on a shooting method. The performances of these two modes have been evaluated by calculating the recharge amount. When the sink term due to the plant roots was included, it was found that the recharge is significantly higher for the case of periodic operation. A physical explanation for the results obtained is proposed. The effect of hysteresis in the water retention curves was simulated using an empirical method. We have found that the total recharge amount in the periodic operation calculated using a mean nonhysteretic curve is very close to that obtained when we include the effect of hysteresis.

The saturated hydraulic conductivity $K$ and the effective porosity $f$ are two important input parameters needed for lateral drain spacing design, as well as some other applications. The technical and economic justification, of most drainage projects, is mainly connected to these two parameters. The current design procedure is based upon calculation of the lateral spacing, using some average values of $K$ and $f$ within the drainage area. The objectives of this study were to introduce a new method for simultaneous estimation of $K$ and $f$ parameters using the inverse problem technique, and to evaluate five different unsteady drainage analytical models of the Boussinesq equation, suggested by different researchers for simultaneous prediction of the parameters. Consequently, five different analytical models for predicting water table profiles were solved, using the inverse problem technique. Each model was then evaluated. A physical drainage model of $2.2\mathrm{m}$ length, $0.3\mathrm{m}$ width, and $0.5\mathrm{m}$ height was established in the laboratory and carefully packed with a sandy loam soil. A perforated drainage pipe of $4.5\mathrm{cm}$ in diameter was installed at the bottom end of the model. Many piezometers were inserted in the soil for spatial and temporal water table monitoring. Different data sets from the experiments and literature were used for model calibration. The newly proposed approach that is based upon measuring water table profiles, at different times, was then evaluated with both constant and variable $f$ . The predicted values of the proposed approach indicated reasonable agreement with the measured data. With variable effective porosity, the method was even more accurate to predict the water table profiles. Using the inverse problem technique, all the analytical models provided good agreement with the measured data. Among these, however, the Topp and Moody model predicted more accurate results than other models.

Presented herein is a procedure for arriving at an optimal design of a homogeneous earth dam laid on an impervious foundation and provided with a drain. The procedure, heavily dependent on variably saturated flow modeling, involves optimizing a multiobjective function comprising a weighted summation of four objective functions, viz., the dam section area, seepage discharge, wetted area of the dam section and the drain area. The design variables considered in the optimization are the upstream and downstream slopes and the drain dimensions. The optimization is carried out subject to the constraints ensuring safe upstream and downstream slopes and sufficient distance between the free surface and the downstream face. Two of the objective functions (viz., the seepage discharge and the wetted area) and the constraints are implicit functions of the design variables. Their values are obtained by employing a numerical model of two-dimensional (vertical plane) variably saturated flow in a homogeneous earth dam. Optimization, conducted by the sequential unconstrained minimization technique procedure is preceded by several runs of the model for various combinations of the design variables. The discrete values of the implicit functions so generated are invoked during optimization to compute the implicit objective functions and constraints. The results are presented in the form of nondimensional design tables/curves. The design procedure is illustrated with the help of few examples.

The man-made development of lands for irrigated agriculture has provided benefits, but in some schemes has resulted in catastrophic problems arising from soil salinization including those in the Aral Sea Basin in central Asia and the Murray-Darling Basin in Australia. Soil salinization is a gradual process and the impact of salt intrusion is noticeable after several years of development, typically ranging from $10\text{to}30\text{years}$ . In central Asia salt water management has historically been based on rules of thumb with mixed results. A coupled surface and ground water model, HYDRO-GW, has been developed with the facilities to model the processes in the root zone and in the unsaturated and saturated subsurface system. The model is applied to the Kashkadarya irrigation area in Uzbekistan to assess the impact of various changes in land and water management practices on reducing salinity levels. Simplified equations have been presented which have potential application for assessing the salinization process and can be used as precursors to detailed assessment using a coupled surface and ground water model.

In this paper an analytical procedure taking into account the nonuniform outflow profile for hydraulic analysis and design of multiple outlets pipelines, is presented. Energy relations are improved based on the average friction drop approach with a simple exponential function, to express the nonuniform outflow concept. To determine friction head losses, the Darcy-Weisbach formula is used here; and the kinetic head change is considered whereas minor head losses are neglected. Several mathematical relationships are also derived for computing extreme pressure heads and their locations of occurrence along the pipeline. The presented method also provides specific lengths of the segments in which the different flow regimes occur along its length. This method simulates pressure and outflow profiles along trickle and sprinkler irrigation laterals and manifolds, as well as gated pipes. The presented technique was applied to several computational examples to clarify its precision for trickle and sprinkler lateral design and the analytical results were compared with those obtained using the numerical step-by-step method. The comparison test for the various design combinations indicated that, the proposed method is found to be sufficiently accurate in all design cases for both trickle and sprinkler lateral design. The analytical development is simple, direct, and easily adaptable to solve hydraulic design problems of various types of single-diameter mutiple-outlet pipelines in different flow regimes and uniform line slope cases. It is preferred to the numerical techniques which need large amounts of execution time and complex computer operations.

A semianalytical model is developed for computing drawdowns in and around a partially penetrating large diameter well. The new model can take into account an unsteady pumping discharge and thus drawdowns during recovery can be computed. This model can also yield the unsteady contributions from well and aquifer storages to the pumped discharge. While developing the model, the flow from the bottom of the well is also accounted.

The present approach adopted for suggesting alternative sustainable land use comprises taking into consideration present land use/land cover, soils, slope, and geomorphology. However, this paper deals with watershed management from a different perspective, by stressing the development of the watershed for agriculture activities; first, by implementing soil and water conservation works. The next step is to suggest alternative sustainable land uses based on soil and water conservation measures, groundwater prospects, land capability, and present land use/land cover in the area. The new approach is found to be very useful, as it takes into consideration basic factors necessary for the overall development and management of the watershed, and ensures stoppage of further degradation of the resources through appropriate soil conservation measures and land uses.

The climate in Georgia and other southeastern states of the United States is considered to be humid and the annual precipitation is usually greater than the annual potential evapotranspiration (ET). However, during several months of the year, supplemental irrigation is needed to prevent yield reducing water stress due to the temporal rainfall variability and sometimes due to long-term droughts. The Priestley-Taylor (PT) equation has been used operationally in Georgia to compute ET for irrigation scheduling because of its simplicity, its general acceptable performance in humid regions, and its limited input requirements. A recent study for a site in the humid southeastern United States found that PT overestimated ET and was less accurate than the FAO-56 Penman-Monteith (PM) among some of the approaches that were evaluated. The objective of this study was to assess the potential improvement that can be achieved by replacing PT with FAO-56 PM in Georgia and other southeastern states in a humid climate. More than 70 weather stations across Georgia are available as part of the Georgia Automated Environmental Monitoring Network. Nine representative sites, including Blairsville in a mountainous area and Savannah in a coastal area, were selected to assess the potential improvements that may be achieved by replacing PT with FAO-56 PM. Each site had at least 10 years of daily records that included minimum and maximum air temperature, solar radiation, wind speed, and vapor pressure deficit. PT underestimated the daily and monthly ET during the winter months in the central and southwestern areas and overestimated the daily and monthly ET during the summer months in the coastal and mountainous areas. For the warm season, i.e., April through September, PT slightly overestimated the cumulative ET in the central and southwestern areas, moderately for the mountainous area and severely for the coastal area. Based on these results, it is anticipated that the use of FAO-56 PM for estimating ET will standardize the ET calculations and improve irrigation efficiency in Georgia, especially for the mountainous and coastal areas.

A linearized form of the Boussinesq equation was solved analytically to predict the water table fluctuation in subsurface drained farmland in the presence of recharge and evapotranspiration (ET). The recharge was assumed to be variable with time and the ET considered decreasing linearly with a decrease in the water table height above the drains. The proposed analytical solution was verified for special cases with the existing solutions. There was a close match between the solutions. Applications of the solution in prediction of the water table height in a drainage system are illustrated with the help of physical examples.

When computing peak discharges for the design of drainage systems using the rational method, it is important to have an accurate value for the rational coefficient $\left(C\right)$ . For steeply sloped watersheds the origin of values of the rational coefficient are unknown and lack even modeling verification. A model that shows the relationship between the rational coefficient and watershed slope was developed for steeply sloped watersheds. Using Horton’s infiltration equation, Manning’s equation, the velocity method for computing times of concentration, and generalized intensity-duration-frequency curves, a model was developed to test the effect of variation of several watershed characteristics on the relationship between slope and the rational coefficient. Analyses with the model showed that both Manning’s coefficient and land use had the greatest effect on the relationship between $C$ and slope. A mathematical function was then developed from data generated from the Horton–Manning model. This model allows $C$ to be estimated for a given slope and a value of Manning’s coefficient for the land cover. A rational coefficient at a 6% slope is also required input. The model was tested using several watersheds with moderate to steep slopes. This relationship should be used to better estimate values of $C$ on steep slopes, and thereby, lead to more accurately hydrologic designs.

The cost of open channels can be minimized by using (1) the optimal design concept; (2) a new geometric shape to substitute for the trapezoidal channels, and/or (3) a composite channel. The channels in which the roughness along the wetted perimeter become distinctly different from part to part of the perimeter are called composite channels. The feasibility of a new cross-sectional shape that has a horizontal bed and two parabolic sides and lined as a composite channel is investigated to substitute for the trapezoidal cross section. The optimal design concept is used to establish the efficacy of the proposed new cross-sectional shape, because it gives the best and unique design of open channels. In optimal design concept, the geometric dimensions of a channel cross section are determined in a manner to minimize the total construction costs. The constraints are the given channel capacity and other imposed restrictions on geometric dimensions. The Lagrange multiplier technique is used to solve the resulting channel optimization models. The developed optimization models are applied to design the proposed and trapezoidal channels to convey a given design flow considering various design scenarios which include unrestricted, flow depth constrained, side slopes constrained, and top width constrained design. Each of these design scenarios again takes into account fixed freeboard, and depth-dependent freeboard cases of design. An analysis of the optimization results establishes the cost-saving capability of the proposed cross-sectional shape in comparison to a trapezoidal cross section.

The slit weir has been recently shown to be effective in measuring very small discharge rates. In this study, the slit weir concept is extended to permit one to measure both very low and very high discharge rates accurately. For this purpose, the hydrodynamic concept of images is adopted to form a bank of multislit weirs in which the individual slit weirs have nearly identical flow characteristics. The discharge coefficient of multislit weirs is determined experimentally using the measured discharges and the corresponding heads causing weir flow. The relationship between the discharge coefficient and the main dimensionless parameters is investigated. It is concluded that the discharge coefficient can be represented solely as a function of the Reynolds number.