Journal of Irrigation and Drainage Engineering

Soil samples were taken from the surface to a depth of 152 cm on 16 irrigated sites and one forested site in the fall of 1983. All but one of the irrigation sites had corn grown on it in four out of the previous five years. Soil types of the irrigated sites were Matapeake silt loam, Sassafras sandy loam, Woodstown sandy loam, Rumford loamy sand, and Evesboro loamy sand. Soil samples were also taken in the fall of 1983 and spring of 1984 from irrigated corn plots where poultry manure and ammonium nitrate fertilizer were applied at a rate of 202 kg/ha of available nitrogen. Nitrate concentrations were lower on the forested site than any of the cropland sites. Nitrate leaching was occurring to some extent on all of the irrigated corn sites. The greatest nitrate concentrations in the soil profile occurred where excessive nitrogen was applied using manure in addition to commercial fertilizer. The site where soybeans had been grown in the previous four growing seasons had lower nitrate concentrations than any of the cropland sites. Nitrate concentrations were lower at greater depths on the corn plots in the spring of 1984 than in the fall of 1983. The most dominant factor affecting fall nitrate profile levels is the total amount of nitrogen applied.

A physically based stochastic watershed model is used to estimate runoff prediction uncertainty for small agricultural watersheds in Hastings, Nebraska. The stochastic nature of the model results from postulating a probabilistic model for parameter estimation and input errors. The key factors assumed to contribute to prediction uncertainty are errors in estimating infiltration parameters and moisture conditions prior to a rainfall event. The error distributions for parameter estimates are inferred from soil survey information, and the error distribution for moisture conditions from a regression between antecedent precipitation indices and measured soil moisture. Comparison of model predicted and observed errors demonstrates that the model is conservative in that it is biased towards overprediction of errors.

Controllers that maintain constant water levels at various locations within irrigation canal distribution systems are used in a number of locations. Most are custom‐developed and tested on a particular system or direct copies of earlier successful versions. Design of such feedback‐control mechanisms is hampered by a lack of information on the dynamic response of the control system to various conditions. In this paper, we examine the dynamic response of one type of mechanical‐hydraulic controller, the dual acting controlled leak system (DACL). Simple transfer‐function equations are developed, from which the stability and dynamic response of the system can be evaluated. These characteristics are evaluated in a companion paper.

Constant water‐level controllers of various kinds are used within many irrigation canal distribution systems. In a companion paper, simple transfer‐function equations were developed for one style of automatic water‐level controller, the dual acting controlled leak system. Such information can be used to study the dynamic response of proposed designs. In this paper, these transfer‐function equations are used to evaluate the stability and dynamic response of the system. It is shown that these systems are always stable. Response times and control decrement are well modeled.

A 3‐in. (7.62‐cm) Parshall flume is installed in a channel and flow rates are measured with lateral flume crest slopes of 0, 3.6, 6.5, 9.0, 13.3, $-3.8\text{,}$ $-4.8\text{,}$ $-7.2\text{,}$ and $-11.8\%\text{.}$ The apparent discharge is compared to the measured discharge for each slope with free outfall conditions. The results indicate that the Parshall flume accuracy is in error approximately 7% at a lateral slope of $\pm 10\%\text{.}$ It is determined that the flow measurement requires a 0.75% adjustment for each 1% of lateral settlement at the flume crest. A method for correcting the apparent discharge with the measured discharge is presented for variable lateral slope conditions.

Performance measures are developed that facilitate analysis of irrigation‐water delivery systems in terms of adequacy, efficiency, dependability, and equity of water delivery. The measures provide a quantitative assessment not only of overall system performance, but also of contributions to performance from the structural and management components of the system. Spatial and temporal distributions of required, scheduled, deliverable, and delivered water are used to calculate the performance measures. These variables may be estimated by a combination of field‐measurement and simulation techniques. The performance measures can be incorporated in an irrigation‐system monitoring program and can provide a framework for assessing system improvement alternatives. They are amenable to decomposition analysis of systems, allowing assessment of trends in performance among distinctly defined subregions or comparison of performance at different levels of system‐network hierarchy. Example applications to systems typical of Sri Lanka and Egypt indicate the usefulness of the measures in system evaluation.

A vapor‐budget technique for measurement of short‐term evaporation from moist, rough surfaces (such as irrigated cropped areas) is developed based on the control volume concept. The control volume is specified so that the difference in the horizontal fluxes of outgoing and incoming atmospheric water vapor equals the evaporation rate. The fluxes are estimated by measuring vertical profiles of wind speed and water vapor. A height of 3 m is sufficient for the present field application. Simplifications used in actual application of the technique are discussed and evaluated. This approach is field tested (square test site, 30 m on each side) and compared with simultaneous water‐budget measurements of the evaporation. The vapor budget is within 12% of the water budget for steady wind direction so long as an average of five profile sets is used for each 15‐min evaporation value. One of the primary advantages of the control volume approach is that it requires measurements only at the periphery, thus providing integrated evaporation values for complex surfaces.

A general infiltration model is derived using a systems approach. The models of Horton, Kostiakov, Overton, Green and Ampt, and Philip are some of the example models that are shown as special cases of the general model. An equivalence between the Green‐Ampt model and the Philip two‐term model is shown. The general model also provides a solution for the Holtan model expressing infiltration as a function of time. This solution of the Holtan model has not been reported in the literature. A first‐order analysis is performed to quantify the uncertainty involved with the generalized model. The general infiltration model contains five parameters. Two of the parameters are physically based and can therefore be estimated from the knowledge of soil properties, antecedent soil moisture conditions, and infiltration measurements; the remaining three can be determined using the least squares method. The model is verified using ten observed infiltration data sets. Agreement between observed and computed infiltration is quite good.