Journal of Irrigation and Drainage Engineering

The discharge coefficient of a lateral opening in a prismatic trapezoidal main channel is determined for subcritical flow conditions from considerations of the conservation of energy and the conservation of momentum. The latter requires an empirical relationship for the local downstream component of the spillage velocity with the local downstream velocity in the main part of the channel. The discharge coefficients obtained by both considerations for a broad range of Froude numbers, combination of both side slopes of 1V: mH with $m=0\text{,}$ 0.5, 1.0, and 2.0, and different lengths of lateral opening are found to be nearly the same. The measured water depth just downstream of the lateral opening is not recommended for use in the numerical determination of the discharge coefficient and the reasons are discussed in detail. The discharge coefficient is found to be related to the upstream Froude number and this relationship is empirically established.

The ARC/INFO geographic information system (GIS) has been combined with the ARX spatial expert system and the SMSP soil moisture‐soil strength model to give the managers of a major Australian army training base the ability to predict damage from exercises. Soil types on the base, their wetness indices, and moisture status are biophysical inputs to the damage prediction model. ARX is used to predict the soil types and wetness indices. These results and antecedent rainfall are then used by SMSP to predict soil moisture and soil strength. ARC/INFO is used to store and map the results. In this paper, we test the accuracy of the soil type and soil moisture predictions. Field trials demonstrated that the soil type predictions are acceptable, but that SMSP is unable to accurately predict soil moistures at individual sites, although predictions over transects of 1.5 km are acceptable.

The effect of irrigation and nitrogen management on ground‐water quality was evaluated for four years on a Sassafras sandy loam Coastal Plain soil. Applying the greatest portion of the nitrogen by side‐dressing and by fertigation were compared. Maintaining optimum soil moisture and partial irrigation (applying one half the water as optimum irrigation) were the water management practices investigated. Nitrate concentrations increased in the ground water for all nitrogen and irrigation‐management practices. The mass of the nitrate leached was related to the drainage volume. In all but one year the largest mass of nitrate was leached during the fall and winter months, when the largest amount of recharge occurs. Very little nitrate was leached during the growing season for any nitrogen or irrigation management practice except for one year when 30 cm of rainfall occurred in August. The mass of nitrate leached during that growing season ranged from 33.9 kg/ha for partial irrigation to 139.0 kg/ha for full irrigation.

Analysis of surface runoff on upland areas requires identification of roughness coefficients. A laboratory study is conducted to measure Darcy‐Weisbach and Manning roughness coefficients for corn, cotton, peanut, pine needles, sorghum, soybeans, sunflower, and wheat residue. Varying rates of flow are introduced into a flume in which selected amounts of residue are securely attached. Roughness coefficients are calculated from measurements of discharge rate and flow velocity. The laboratory data are used to derive regression equations for relating roughness coefficients to Reynolds number and either percent residue cover or residue rate. Separate equations are developed for Reynolds number values from 500 to 20,000, and from 20,000 to 110,000. Generalized equations are presented for estimating roughness coefficients for other residue materials not used in this investigation. Accurate prediction of roughness coefficients for residue materials will improve our ability to understand and properly model upland flow hydraulics.

The determination of vertical ground‐water leakage is particularly important for irrigated agriculture where adequate drainage is required to prevent high water tables and related problems of salt buildup and waterlogging. This paper presents results from six different methods that were used to estimate vertical leakage for the Nile Valley in Egypt: (1) Calculation by Darcy's law; (2) rate of water table decline; (3) water budgets; (4) leaky aquifer test; (5) analytical solution; and (6) numerical ground‐water modeling. Considering the basic differences in the underlying assumptions of each method, the results were in relatively good agreement. The average vertical leakage determined for the entire Nile Valley was about $0.65ċ{10}^{-3}m/\mathrm{day}\text{.}$ This value is quite low, much lower than previously reported elsewhere in the literature. This low rate of natural drainage results in high water tables that cannot be lowered without the use of artificial drains. A salt concentration balance indicated that given the small value of vertical leakage in the absence of artificial drainage, the salinity of the ground water in the Nile Valley may increase to the point where it would hinder crop production.

Shear strength tests are conducted in a model of a water, well on four, commercially available, granular bentonite well sealing products. The time dependency of annulus seal shear strength and resistance to piping is examined. Dry bentonite samples are placed through water in the annular space between the casing and borehole and allowed to set for a specified time. Pressure is applied at a constant rate to one end of the clay seal until shear or piping failure occurs. Pressure and seal displacement data are recorded on a digital data acquisition system. Annulus seal strength and resistance to piping increase with setting time. Ultimate shear strength is reached in 72 to 120 hours after placement. Differences between this laboratory scale study and field scale applications are discussed.

Water surface temperatures of Lake Okeechobee are measured using automatic picture transmission (APT) thermal infrared data from the National Oceanic and Atmospheric Administration (NOAA) polar‐orbit satellite advanced very high resolution radiometers (AVHRR). The cloud‐free APT thermal infrared data are acquired during the months of May and December, 1989. The spatial temperature variation of Lake Okeechobee is investigated. The correlation of pan evaporation with water surface temperature and solar radiation is studied. A regression model of evaporation based on satellite‐derived water temperature, ground‐measured water temperature, and solar radiation is developed. The results indicate that pan evaporation is highly correlated with the temperature and solar radiation, and that the satellite‐derived lake temperatures have a better correlation $(r^2=0.84)$ to the pan evaporation than did the ground‐point measurements $(r^2=0.68)\text{,}$ especially for afternoon situations. This implies that the NOAA polar‐orbit satellite APT data can be used to assess lake surface temperature and to estimate lake evaporation.

Laboratory experiments are conducted to evaluate the hydraulic characteristics of several circular flumes. The circular flume is a simple and low‐cost water measuring device constructed from two pieces of pipes, one installed vertically inside the other. The ratio of the diameter of the inner column to the flume itself is approximately one‐third. The presence of the inner column reduces the cross section of the flow, creating a critical flow condition. A gauge is installed at the upstream side of the inner column to measure the depth of the water upstream of the critical flow section. The reading on the gauge is directly related to the flow rate. The results of the laboratory experiments showed that the circular flumes can be successfully used to measure the flow rate in open channels. Based on the results of the laboratory experiments, a computer model is developed for the calibration of circular flume. The computer model is tested using data from flumes installed in the laboratory and in the field.

Farmers who receive water from irrigation districts prefer deliveries that are flexible but accurate in frequency, rate, and duration. To study how well an irrigation district was able to match deliveries to farmers' orders and to account for deliveries, water deliveries along two monitored lateral canals were compared to the orders and bills for those events. The data showed that farmers complied with the official ordering policy about half the time, and were more likely to follow the policy during the late summer than during the spring. No corresponding bill existed for 16.8% of deliveries. Deliveries occurred on average about a half day later than ordered by farmers, and 72% occurred within one day of the ordered date, which is within the latitude allowed under district policy. One‐third of the deliveries ordered within the official policy had a mean flow rate plus or minus 10% of that ordered; 87% had durations plus or minus 10% of that ordered. For 43% of deliveries that had corresponding bills, billings were for flow rates within 10% of the measured mean; 50% were billed for durations within 10% of measured values. Further research is needed to determine the economic and environmental effects of inaccurate deliveries, and thus determine incentives to improve performance.

A modification to the double‐sweep routine commonly used to solve the continuity and momentum equations for surface irrigation models has been developed to allow the solution of one global parameter at each time step. Coefficients for the double‐sweep routine are presented for the Kostiakov infiltration parameter k, the Manning n, and the time step $\delta t\text{.}$ A procedure is also developed for determining the entire infiltration relation (e.g., both Kostiakov exponent a and constant k) during advance. It is shown that this estimated relation is subject to errors caused by assumptions regarding the volume of water in surface storage near the advancing tip. Other errors in field measurements also significantly impact estimates of infiltration and roughness made during advance. Such errors are particularly problematic when trying to use estimates during advance to predict conditions at the end of the irrigation, as is the case for feedback control.

Accurate measurement of applied water volumes in surface irrigation fields is important towards improving irrigation efficiencies and management of the surface irrigation system. In flood irrigated fields, applied water may be supplied through spiles, or turnout pipes connecting the field directly to the supply canal. A simple measurement of head loss across the turnout pipe is all that is necessary to estimate onflow rates when the total head loss coefficient for the spile is known. The total head loss coefficient is primarily dependent on the entrance and exit head losses, and entrance head losses depend on the orientation of flow in the canal relative to the spile. For spile flows perpendicular to the axis of canal flow, the entrance head loss is greater than traditionally assumed. This paper reports results of field calibration of spiles having a supporting bracket for the cover plate, perpendicular flow and into which flow develops a sizable vortex that dissipates energy. A calibration curve relating head loss to spile discharge is developed from the field data.