Journal of Irrigation and Drainage Engineering

The sensitivity of risk cost to hydrologic parameters is evaluated to determine the feasibility of risk analysis as an alternative to existing requirements for rehabilitation at small dams in Missouri. The sensitivity to probable maximum precipitation (PMP) exceedence probability is greatly reduced by interpolating rainfall exceedence probabilities between the PMP and the rainfall depth that initiates failure, instead of the PMP and the 100‐year return period depth. This approach also reduces by a small amount the sensitivity of risk cost to the assumed probability‐distribution function. Variations in the time distribution of rainfall and the antecedent moisture condition (AMC) have a large influence on risk cost. Probabilistic combinations of the three AMC classes result in risk costs within 13% of the risk cost using AMC II, the “average” condition. Of the three dam‐breach parameters evaluated, risk cost is most sensitive to breach‐development time and least sensitive to breach side slopes.

This paper develops a nonlinear optimization model for the design and management of drip irrigation systems. Decision variables are: pipe diameter, pipe length, number of emitters in each lateral, number of laterals in a manifold, total number of subunits, number of subunits operating simultaneously, irrigation time per set, and emitter discharge. The model is solved using the GAMS‐MINOS package. To illustrate the capability of the model, an orchard field of pears is selected. The price effect on the results of the model shows that the cost of the system and its operation are relatively small compared with the benefit obtained. The model shows that the results do not yield the minimum cost of the system when the marginal benefit is greater than the marginal cost.

The artesian wells flowing uncontrolled in many parts of Florida have caused a serious saline‐contamination problem. Locating these wells by ground search is made difficult by the urbanized or reforested condition of formerly agricultural land. Both ground‐based color infrared (GCIR) and aerial color infrared (ACIR) photographs were taken for analysis of the spectral reflectance of land‐surface features that can provide information for detection of flowing wells. Both GCIR and ACIR showed similar patterns of spectral reflectance for the same component class of land‐surface features. Well‐site soil had a higher spectral reflectance than soil not associated with a well. Well water had higher spectral reflectance than natural pond and ditch water. The spectral reflectance patterns of vegetation types included in this study all have two peak‐intensity values. The vegetation types had differences in spectral reflectance magnitude except for leather fern and cattail (similar to each other) and St. Augustine grass and cucumber (also similar to each other). The combination of green and red channels of spectral reflectance video‐digitized from ACIR photography appears to be useful for classifying land‐cover types and distinguishing flowing wells.

This paper deals with the development of equations to predict water‐table geometry for flow toward a drain ditch or reservoir whose water surface level is subject to a sudden specified drop. A closed function representing the water‐table geometry is obtained for the classical potential formulation of the problem under initial and boundary conditions related by means of perturbation method. The first‐order term of this closed function is identical to the solutions published by Higgins (1980), for large values of time, by Stallman (1962), and by Bear (1974). The inclusion of the second and third order terms improved the accuracy considerably. The water table profile found as a function of only one variable is predicted for various $\mu =d_0/h_0$ values, ranging between $-1$ and 0. Temporal and spatial variations of the water table are obtained for various μ values. The comparison of the present results with those of previous works is also presented.

A desktop method is presented to analyze or size detention ponds and outlet facilities. Different initial conditions are allowed. When used for design purposes, the method determines the size of the outlet facility for a given pond. However, if the pond is inadequate to meet the design criteria set, the method determines the required stage‐storage relationship. The method is based on the usual reservoir‐routing technique that couples the hydrologic‐storage equation with an expression describing the flow through the outlet structure. The equations are rearranged in terms of several dimensionless parameters. A computer model is used to solve the governing equation using a finite difference scheme. The numerical results obtained in terms of systematically chosen combinations of the governing dimensionless parameters are generalized based on the concept of similarity. The generalized results are presented in chart form. Sample applications of these charts are illustrated.

A numerical model is developed to simulate unsteady flow in open‐channel networks. The model is an improvement over currently available network models in that the effects of hydraulic structures and leakage through the riverbed are incorporated. In addition, a matrix solution method is developed that takes advantage of the banded nature of network problems and, in some cases, reduces the solution time drastically. A method of reducing the bandwidth of the coefficient matrix is presented. Verification by comparison to other solution methods shows close agreement. The computation time using the banded matrix solution routine is compared to a commonly used method for reducing solution time to determine the conditions under which the banded matrix solution is faster. The model presented is useful in simulating gate‐operating rules, analyzing effective flood‐control measures, and comparing water‐delivery schemes over large distances where there is significant loss to leakage.

Contamination of ground water by nitrogen leached from fertilizer on irrigated soils is related to the quantity of nitrate‐N $({\mathrm{NO}}_3^{-}‐N)$ present, the leaching potential based on soil texture and percent depletion of available soil water in the root zone, and the amount of water entering the soil profile. Research and demonstration projects in the central Platte valley of Nebraska have shown that ${\mathrm{NO}}_3^{-}‐N$ leaching is influenced by both irrigation and fertilizer‐nitrogen (N) management in corn production. Scheduling irrigation according to available soil‐water depletion can reduce deep percolation to a certain extent. Additional reduction in deep percolation can be achieved by improving efficiency of water application, particularly on furrow irrigated fields. Testing for ${\mathrm{NO}}_3^{-}‐N$ in irrigation water and soil can provide for substantial reductions in fertilizer N application, if residual levels in the soil are high, or if considerable ${\mathrm{NO}}_3^{-}‐N$ will be applied with irrigation water. Grain yields were not appreciably affected by the use of these management practices, while in most cases input costs for fertilizer nitrogen and irrigation water were reduced.

The National Weather Service River Forecast System (NWSRFS) model is used to generate 30 years of sequential streamflows for Laguna de Santa Rosa in northern California. The model is calibrated by simulating 11 years of daily streamflows for a 12.5 sq mi subwatershed in the upper portion of the Laguna de Santa Rosa watershed. Daily streamflow data for this subwatershed are available at a USGS gauging station. Daily precipitation values are obtained as weighted average of precipitation data at two stations in the vicinity. These are converted into a six‐hourly time series using average distribution patterns for six‐hourly precipitation. Daily lake evaporation values are estimated using regression analyses with monthly pan evaporation data for nearby stations, daily average distribution patterns, and applicable pan coefficients. These values are converted into evapo‐transpiration demand by the model using specified evapotranspiration adjustment factors. In spite of these approximations and adjustments, the calibration is satisfactory. The calibrated model is used to generate and combine daily streamflows for three tributaries of the Laguna de Santa Rosa.

The effect of rainfall intensity on reaeration has not been adequately reported in the literature. It is clear that rainfall causes surface turbulence and increases reaeration, and a significant linear relationship (r = .97) was found in this study between rainfall intensity and K_L(20), the corrected mass transfer rate at 20°C, an initial oxygen concentration of zero, and one atmosphere of pressure. Experimental pool data indicate rainfall can be an important contributor to reaeration, particularly in stagnant aquatic systems where oxygen saturation deficits are large. Calculations illustrating the reaeration effect of various rainfall intensities at several undersaturated sites in the Florida Everglades Water Conservation Area 2A (WCA-2A) support this conclusion and show reaeration rate increases of 23 to 103 times over normal field rates at rainfall intensities from 0.64 to 2.54 cm/hr, respectively.