Journal of Irrigation and Drainage Engineering

Net radiation $\left(R_n\right)$ is the main driving force of evapotranspiration (ET) and is a key input variable to the Penman-type combination and energy balance equations. However, $R_n$ is not commonly measured. This paper analyzes the impact of 19 net radiation models that differ in model structure and intricacy on estimated grass and alfalfa-reference ET ( ${\text{ET}}_o$ and ${\text{ET}}_r$ , respectively) and investigates how climate, season and cloud cover influence the impact of the $R_n$ models on ${\text{ET}}_o$ and ${\text{ET}}_r$ . Datasets from two locations (Clay Center, Nebraska, subhumid; and Davis, California, a Mediterranean-type semiarid climate) were used. $R_n$ values computed from the 19 models were used in the standardized ASCE-EWRI Penman-Monteith equation to estimate ${\text{ET}}_o$ and ${\text{ET}}_r$ on a daily time step. The influence of seasons on the estimation of $R_n$ and on estimated ${\text{ET}}_o$ and ${\text{ET}}_r$ was investigated in winter (November–March) and summer (May–September) months. To analyze the influence of clouds on the impact of $R_n$ models, relative shortwave radiation $\left(R_{\text{rs}}\right)$ was used as a means to express the cloudiness of the days as: $0\le R_{\text{rs}}\le 0.35$ for completely cloudy days; $0.35<R_{\text{rs}}\le 0.70$ for partially cloudy days; and $0.70<R_{\text{rs}}\le 1.0$ for clear sky days. The performances of $R_n$ models showed variations at the same location and between the locations for the same model based on methods used to calculate various model parameters. The most significant impact of $R_n$ on estimated ${\text{ET}}_o$ and ${\text{ET}}_r$ was related to the methods used to calculate atmospheric emissivity $\left(\epsilon \right)$ rather than methods used to calculate clear sky solar radiation $\left(R_{\text{so}}\right)$ or cloud adjustment factor $\left(f\right)$ . $R_n$ models that used average air temperature to compute $\epsilon$ and an estimated $f$ resulted in good performances at both locations. Empirical models that assumed $f=1.0$ showed poor to average performances at both locations. While model performances varied based on methods used to calculate $R_{\text{so}}$ , $f$ , and $\epsilon$ , there were significant seasonal variations in performances of models that calculated $\epsilon$ as a function of actual vapor pressure of the air $\left(e_a\right)$ . The seasonal variations in performances of these models were greater under subhumid climate at Clay Center than in semiarid climate at Davis, Calif. The models that calculated $\epsilon$ as a function of $e_a$ performed better under completely cloudy days than on other days, more so at Clay Center. Methods used to calculate $\epsilon$ have a significant impact on the $R_n$ model performance, especially in unstable climatic conditions such as at Clay Center where there are frequent and rapid changes in climatic variables in a given day and throughout the year. The results of this study can be used as a reference tool to provide practical information on which method to select based on the data availability for reliable estimates of daily $R_n$ relative to the ASCE-EWRI $R_n$ method in subhumid and semiarid climates similar to Clay Center, Neb. and Davis, Calif.

Vegetable production areas are intensively managed with high inputs of fertilizer and irrigation. The objectives of this study were to evaluate the interaction between N-fertilizer rates and irrigation scheduling using soil moisture sensor irrigation controllers (SMS) on yield, irrigation water use efficiency (IWUE) of bell pepper cultivated under plastic mulch and drip irrigation. Treatments included three irrigation scheduling and three N-rates (176, 220, and 330 kg/ha). Irrigation treatments were: ${\text{SS}}_{10}$ , water application controlled by SMS-based irrigation set at 10% volumetric water content (VWC) which was allotted five irrigation windows daily and bypassed events if the soil VWC exceeded the established threshold; ${\text{SS}}_{12}$ , threshold set at 12% VWC; and TIME, control with irrigation being applied once a day similar to grower irrigation management. Marketable yields ranged between 16 and 29 Mg/ha. The SMS treatments reduced the applied irrigation in 7 to 62% compared to TIME treatment without reducing yield. The treatments ${\text{SS}}_{10}$ and ${\text{SS}}_{12}$ reduced nitrate leaching by 25 to 73% compared to TIME treatment.

This paper presents a practical method to manage soil salinity and yield in order to obtain maximum economic benefits. The method was applied to a study area located in the southeastern part of the Arkansas River Basin in Colorado where soil salinity is a problem in some areas. The following were the objectives of this study: (1) generate classified maps and the corresponding zones of uncertainty of expected yield potential for the main crops grown in the study area; (2) compare the expected potential productivity of different crops based on the soil salinity conditions; and (3) assess the expected net revenue of multiple crops under different soil salinity conditions. Four crops were selected to represent the dominant crops grown in the study area: alfalfa, corn, sorghum, and wheat. Six fields were selected to represent the range of soil salinity levels in the area. Soil salinity data were collected in the fields using an EM-38 and the location of each soil salinity sample point was determined using a global position system unit. Different scenarios of crops and salinity levels were evaluated. Indicator variograms were constructed for each scenario to represent the different classes of percent yield potential based on soil salinity thresholds of each crop. Indicator kriging (IK) was applied to each scenario to generate maps that show the expected percent yield potential areas and the corresponding zones of uncertainty for each of the different classes. Expected crop net revenue for each scenario was calculated and all the results were compared to determine the best scenarios. The results of this study show that IK can be used to generate guidance maps that divide each field into areas of expected percent yield potential based on soil salinity thresholds for different crops. Zones of uncertainty can be quantified by IK and used for risk assessment of the percent yield potential. Wheat and sorghum show the highest expected yield potential based on the different soil salinity conditions that were evaluated. Expected net revenue for alfalfa and corn are the highest under the different soil salinity conditions that were evaluated.

Parshall flume tests were conducted to determine the sensitivity on indicated flow rate resulting from improperly located staff gauges and differing entrance conditions to the flume. An acrylic 2 ft (61 cm) Parshall flume was constructed to specified design dimensions and installed level in an 8 ft (243.8 cm) wide testing flume at the Utah Water Research Laboratory. Multiple stilling wells and two sliding point gauges were used to determine the head measurements from stilling well ports and water surface elevations along the wall and down the center of the converging approach section of the flume for multiple flow rates. The recommended radius wingwall, a 45° wingwall and no wingwall were also tested with and without offsets at the entrance to the flume. The laboratory tests indicated that head measurements taken at locations other than the location specified by Parshall $\left(2/3A\right)$ can produce up to 60% errors in the flow measurement. Additionally, the lack of wingwalls and the incorrect design and installation of wingwalls cause standing waves to form in the throat of the flume which also affects flow measurement accuracies.

Circular drop manholes are widely employed in steep urban drainage systems. Drop manholes may lead to poor hydraulic conditions if their energy dissipation is inadequate. The dominant hydraulic features of drop manholes depend on the flow regimes, characterized in terms of the dimensionless impact parameter. Depending on the latter parameter, the energy dissipation can vary within large limits, affecting thereby the downstream flow features. Also, the water pool depth inside the manhole and the air entrainment have been studied in terms of both the hydraulic and geometric parameters. Moreover, the conditions for which a drop manhole generates flow choking at its inlet or outlet have been investigated. Empirical equations for practical manhole design are provided. The importance of suitable manhole aeration is highlighted.