Journal of Irrigation and Drainage Engineering

Furrow shape information is required for modeling and evaluating furrow irrigation. Currently used shape models assume the furrow perimeter is rigid so that only the flow depth increases with capacity. Actual furrow perimeters are not rigid and may widen as their capacity increases. If the furrow width increases proportionally with flow depth, the flow cross‐sectional shape remains constant and only the size increases with capacity. This constant‐shape model results in simple generalized relationships between the hydraulic and geometric parameters, which simplifies analysis of the complicated interactions that occur during furrow irrigation. The two shape models are compared conceptually and against field measurements. The rigid‐perimeter model better matches field‐measured furrow shapes and is easier to rationalize conceptually. However, both models match the important relationships between furrow geometric parameters and hydraulic parameters equally well. The most important relationship between flow area and uniform flow section factor is insensitive to both the model and shape. The predictions of both models are more sensitive to the furrow top width‐to‐flow depth ratio than to shape.

In present hydrologic practice, it is generally recognized that the shallow overland sheet flow through the upper portions of a watershed is very slow, and that runoff gradually accelerates as it concentrates into gullies and, finally, streams. In this paper, the fractal concept is applied by testing whether a watershed's stream‐and‐swale system branches in a characteristic pattern that repeats itself into successively finer but similar segments. Commonly used geometric, geomorphologic, hydraulic, and hydrologic equations are combined to reduce the number of parameters. Kinematic wave equations are then developed for the runoff movement in the channels, and integrated to obtain an equation for the time of concentration.

Level‐basin irrigation has been studied at two sites in western Colorado for seven years. This paper describes changes in the salinity profile in the root zone under level‐basin irrigation. On soils with initially low salt content, salt concentrations in the soil profiles increased under level‐basin irrigation, because the irrigation efficiencies are greater than for prior practices. Conversely, good water management and uniform applications on level basins lowered the salt content in the top 60 cm of soils with high initial salt content. After a few years of level‐basin irrigation with Colorado River water, in either case, the salt content equilibrated at levels that do not restrict growth of common field crops.

The Surface Mining Control and Reclamation Act of 1977 requires evaluation of pre‐ and postmining hydrology for design of runoff and sediment control structures on mined land. The runoff‐curve‐number (CN) method as developed by the Soil Conservation Service has been routinely used to predict rainfall‐runoff relationships for ungauged watersheds disturbed by surface mining and reclamation. However, lack of sufficient rainfall‐runoff data from reclaimed surface mines in the Eastern United States has prevented accurate calibration of CN values for those areas. Median CN values calculated from rainfall‐runoff data from reclaimed surface mines in central Pennsylvania indicate that the currently accepted methods for estimating CN values underestimate total runoff from the standard design storm by as much as 50%. Median CN values range between 83 and 88 with CN values as high as 99 for soils with antecedent moisture condition classification III. It is recommended that hydrologic soil group D be adopted for sandy‐loam and silty‐loam postmine soils and used with the appropriate postmine land use to calculate CN values for reclaimed‐surface‐mine watersheds.

Hydraulic conductivity is an important parameter in the design of subsurface drainage systems. Hydraulic conductivity from field investigations in alluvial soils shows a high spatial variability. The objective of this study was to show how kriging, a linear unbiased estimator, can improve the representation of hydraulic conductivity for subsurface drainage design. The spatial autocorrelation structure used in the kriging estimations was represented by the semivariogram of 3,488 hydraulic conductivity measurements done on a 35,000 ha area in the Nile Delta of Egypt. Results showed that variations of the hydraulic conductivity were isotropic, with a high nugget effect, and a range of influence of 5.6 km. Kriging allowed clear identification as contour maps that could be used to determine blocks of homogeneous hydraulic conductivity. For the 25 drawing areas studied in this paper, differences in design hydraulic conductivities calculated from the kriged and measured values ranged from 114.0% to $-40.3\%\text{.}$

For a given level of irrigation uniformity there is a trade‐off between the amount of water applied and the area that receives some target amount of water. If the distribution of infiltrated depth is known (e.g., a normal distribution), then the relation between these two can be quantified. In addition, the average amount in that area not receiving the target amount can be determined with the relations provided herein. It is shown how these relations can be used with combination of variance techniques to make decisions on the gross amount of water to apply to an irrigation set, a field, a farm, or a group of farms based on target amounts to be applied and the area that is to receive this desired amount.

Irrigation is becoming increasingly important in midwestern agriculture. Yet, since the Midwest has traditionally not been heavily irrigated, relatively little has been known about present irrigation water use and irrigation scheduling and efficiency in that region. Questions are arising with greater frequency about irrigation in the subhumid climate of the Midwest, and about the impact of irrigation water use on regional water resources. Biweekly and total irrigation amounts and irrigation‐scheduling practices were monitored at representative sites in central Illinois during the 1988 and 1989 growing seasons. Soil water‐holding capacity (expressed as average field capacity in the root zone) correlates well with total irrigation water use, suggesting that irrigation farmers largely determine their irrigation applications based on some understanding of the water‐holding capacity of their soil. Total irrigation water use varies with weather conditions; year‐to‐year variations are greater than variations from farmer to farmer within any single year. Surprisingly little variation in total irrigation applications is evident between different crop types. Irrigation practices vary because of individual farmer idiosyncracies; these variations have no apparent relation to the physical controls of weather, crop type, or soil type. Irrigation farmers are applying irrigation water on corn and soybean crops in amounts and timing that are in general alignment with evaporative demand and with rainfall. The irrigation sites observed in this study may serve as an analogue for gaining greater understanding of irrigation throughout the Midwest.

This paper presents optimality conditions tor a parabolic‐eanal design accounting for freeboard and limitations on velocity and canal dimensions. Freeboard may be based on a design discharge or specified as a function of flow depth, which is more general. The analysis reveals that the side‐slope parameter for the best hydraulic section is 0.514, not 0.7 as reported in the literature for parabolic canals. The aforementioned hydraulic considerations tend to shape the optimal canal narrow and deep. A formulation for the minimization of the sum of excavation and lining costs favors slightly wider canals when the ratio of cost of lining to cost of excavation is high. The necessary conditions for minima are stated analytically. It is shown that the optimal side‐slope parameter of 0.514 sets an upper bound with the implication that no optimal parabola can be wider than the classical best section for a given discharge. Design aids in the form of tables are presented.

Data from rainfall simulation experiments conducted in Arizona and New Mexico were used to identify relationships between total suspended sediment (TSS) concentrations (kg/ha/mm of runoff) in runoff and concentrations (kg/ha/mm of runoff) of total phosphorus (TP), total volatile suspended sediment (TVSS), and total nitrogen (TN). The units of kg/ha/mm of runoff are equivalent to mg/l divided by 100. Data were collected from pinyon‐juniper, ponderosa pine, short grass prairie, creosote bush, and bottomland vegetation types. Lumping data from these five vegetation types yielded a relationship for total phosphorus of $\mathrm{TP}=0.0013(\mathrm{TSS}{)}^{0.83}$ with a linear correlation coefficient of $r=0.77\text{.}$ The relationship for total volatile suspended sediment was $\mathrm{TVSS}=0.274(\mathrm{TSS}{)}^{0.72}$ $(r=0.91)\text{.}$ A poor relationship was found for total nitrogen with $\mathrm{TN}=0.008(\mathrm{TSS}{)}^{0.15}$ (with $r=0.11)\text{.}$ These relationships were validated using data from other rainfall simulation experiments and naturally occurring ephemeral streamflow in southern New Mexico.

Methods of measuring volumes of water withdrawn from the Snake River and its tributaries and pumped through closed‐conduit irrigation systems were needed for equitable management of and resolution of conflicts over water use. On the basis of evaluations and field tests by researchers from the University of Idaho, Water Resources Research Institute, Moscow, Idaho, an impeller meter was selected to monitor pumpage through closed‐conduit systems. In 1988, impeller meters were installed at 20 pumping stations along the Snake River between the Upper Salmon Falls and C.J. Strike Dams. Impeller‐derived pumpage data were adjusted if they differed substantially from ultrasonic flow‐meter‐ or current‐meter‐derived values. Comparisons of pumpage data obtained by ultrasonic flow‐meter and current‐meter measurements indicated that the ultrasonic flow meter was a reliable means to check operation of impeller meters. The equipment generally performed satisfactorily, and reliable pumpage data could be obtained using impeller meters in closed‐conduit irrigation systems. Many pumping stations that divert water from the Snake River for irrigation remain unmeasured; however, regression analyses indicate that total pumpage can be reasonably estimated on the basis of electrical power consumption data, an approximation of total head at a pumping station, and a derived coefficient.

Improved equations are presented tor calculating correction factors for the United Nations Food and Agriculture Organization Irrigation and Drainage Paper Number 24 (FAO‐24) grass reference evapotranspiration methods. Equations duplicate tables of correction factors contained in the original FAO‐24 report and simplify development of computer programming code. Correction factors are included in the original FAO‐24 publication to “correct” reference evapotranspiration estimates for varying climatic conditions of windiness, humidity, and sunshine. Equations are developed in this study using regression models where independent values are comprised of various transformations of secondary parameters used in the Food and Agriculture Organization (FAO) tables.

The dam site selector (DSS) expert system (ES) is an aid for rating an area for use as a dam site and reservoir. The program is not intended for rating sites where the dam height is greater than 50 ft (15.2 m), more than 2500 acre‐ft $\left(3\text{,}075\text{,}000m^3\right)$ of water would be impounded, or the dam and reservoir would have a C hazard classification. It uses the rules of thumb used by an expert for determining potential reservoir sites. It was developed for use in a graduate‐level hydrology design course and has been used in undergraduate and graduate soil and water conservation courses at Purdue University and has also been used by various planning agencies. The features that make DSS a useful educational tool are examined as well as its data requirements and limitations.

A direct identification technique based on linear weighted regression analysis is presented to estimate the power function parameters. The computations required are simple, and a hand-held calculator might be sufficient. Parameters of the power function are obtained when the function is fit to published infiltration and soil moisture–pressure head data by: (1) The standard log-linear technique; (2) the proposed linear weighted regression; and (3) a nonlinear least-squares procedure. The described method was found to be superior to the log-linear technique. Athough the linear weighted least-squares method does not fit the experimental data as well as the nonlinear regression, the simplicity and rapidity of the described method compared to a nonlinear procedure makes it an attractice alternative for estimating the power function parameters.

Considering different fixed and operating-cost components of water production, a mathematical model has been developed for optimum spacing of shallow tube wells in relation to channel lining and conveyance losses. The model is solved for light-textured soil and shallow aquifer condition of North Bihar, India using the existing market rates of materials. However, the mathematical concept is universally applicable. Optimum spacing of 300 m is found when 70% of the channel is lined. The cost of water production for the study worked out to a minimum of Rs.41.08 (one U.S. dollar equals approximately 18.50 Indian rupees [Rs.]) per 100 m³ of volume. For unlined channel, an optimum spacing of 140 m is found for which the cost of water production comes to Rs.41.93 per 100 m³ of volume.

An inspection of the original study that developed the rating equations for Parshall flumes indicates that the calibration tests were not conducted for as low of flows as currently recommended for their use. An experimental investigation for one flume size indicates that the rating equations significantly overpredict discharge at low flow rates. The discrepancy can be as much as 25% for the range of flows recommended. An alternate rating equation is developed for use with these low flow rates.