Journal of Irrigation and Drainage Engineering

Evapotranspiration data from three commercial atmometers ($E{T}_{0-\mathrm{A}}$) were compared over an irrigation season for instrumental precision, and compared against Penman-Monteith ($E{T}_{0-\mathrm{PM}}$) data from an automatic weather station for accuracy. The effects of using contrasting $E{T}_0$ estimation methods on the timing and amounts of water application were then evaluated by using an irrigation-scheduling water-balance computer model. The $E{T}_{0-\mathrm{A}}$ data were statistically analyzed using linear regression, coefficients of determination ($R^2$), root-mean-squared error (RMSE), mean bias error (MBE), and the $t$-test. The study found that the $E{T}_{0-\mathrm{A}}$ data from individual atmometers were closely correlated both with each other and with the $E{T}_{0-\mathrm{PM}}$ data ($R^2=0.68–0.90$). On the basis of water-balance modeling, using an atmometer for scheduling irrigation on potatoes would have resulted in a very similar distribution of irrigation events, but 15% more water would have been applied over the season compared to using $E{T}_{0-\mathrm{PM}}$ data. Day-to-day measured $E{T}_{0-\mathrm{A}}$ values may differ from $E{T}_{0-\mathrm{PM}}$ as the atmometer can only be read to $\pm 0.5\mathrm{mm}$ accuracy, but when averaged over the period of a typical irrigation cycle these errors are reduced and the impact on the irrigation schedule is small. The study suggests that atmometers would be appropriate for scheduling deep-rooted irrigated crops in humid regions with long growing seasons in which the irrigation interval is not less than 5–7 days.

A procedure is introduced to estimate daily solar radiation from maximum and minimum temperatures for areas where solar radiation is either unavailable or unreliable. The procedure uses historical temperature data for spatial and temporal self-calibration. The results from the self-calibrated procedure are compared with measured solar radiation data in one coastal and two interior regions. The self-calibrated procedure is also compared with an equation presented by Thornton and Running that also uses historical maximum and minimum temperature to estimate solar radiation. The comparison between measured and predicted solar radiation values for several years shows that the self-calibrated method can predict solar radiation with error ranging from 0–3%. The error from the Thornton and Running equation ranges from 7–11%. In general, the Thornton and Running equation tends to overestimate solar radiation especially for overcast sky conditions such as monsoon season.

Anwar’s friction correction factor, $G$, and the average correction factor, $G_{\mathrm{AVG}}$, are used to compute the head loss resulting from friction and to calculate the required inlet pressure in horizontal tapered sprinkler laterals with multiple outlets. As a result of their being developed by using a discrete outflow model, which consists of a finite number of operating outlets, the previous approaches for determining these factors required tables or the solving of relatively complex formulas that may be cumbersome. In addition, use of these factors is subject to certain errors when the Darcy-Weisbach equation is used because they are developed assuming a constant friction factor along the lateral. In this study, considering the continuous flow-rate variation concept, at first, two simple and direct equations for calculating the $G$ and the $G_{\mathrm{AVG}}$ factor were developed. The proposed equations are then adjusted for the case in which the variability of the friction factor should be considered. The validity of suggested equations is demonstrated by an example and compared with the accurate numerical solution.

This work examines rainfall-runoff behavior and determines the reference curve numbers (CNs) on a highly weathered tropical clayey soil (Oxisol, or “latosol”) in Brazil for three conditions: (1) bare soil; (2) sugarcane with replanting each three years; and (3) near-permanent sugarcane without replanting. It compares findings with U.S. experiences and handbook values. Curve number and runoff behavior changed markedly after the seventh month of growth, and the profound influences of cover are clearly demonstrated. A surprising finding was that though these Oxisols are quite rich in clay, the rainfall-runoff results suggest that they should be classified as Hydrologic Soil Group B.

In this paper, a methodology is proposed for identifying the best equations for estimating the time of concentration (ToC) of watersheds located in a specific geographic region. The methodology primarily works on the basis of modifying available equations to minimize their bias for any particular region of interest. In the methodology, for each watershed, the ToC considered as “reference” is determined using the Natural Resource Conservation Service (NRCS) velocity method employed in the TR-55 model and geographic information systems data. Moreover, a ranking-based selection strategy is developed for evaluating and selecting the best performing methods. In a case study, the proposed methodology is applied to 72 watersheds and subwatersheds in Khorasan Razavi province, Iran. According to the study, the California equation (with a correction factor of 1.715), the Kirpich (Tennessee) equation (with a correction factor of 1.721), and the Arizona DOT equation (with a correction factor of 1.126) have the best performance for the selected watersheds.

Discharge from a $200{\mathrm{m}}^2$ permeable modular concrete paver test section on an atypical 6.0 to 7.4% slope and in an active roadway was monitored concurrently with an adjacent reference conventional asphalt section in Auckland, New Zealand. The permeable modular pavement (PMP) underdrain water quality had consistent total suspended solids (TSS), zinc, and copper event mean concentrations that were statistically less than the reference asphalt runoff. Event mean concentration (EMC) distributions ranged over only a few μg/L, or mg/L from the PMP underdrain versus more than an order of magnitude for some parameters from the reference asphalt. Runoff volume control by the PMP contributed to substantial pollutant mass loading differences despite clay subsurface soils. PMP joint and bedding material were identified as the likely source of most pollutants in the underdrain discharge that might be avoidable with a construction specification for well-washed materials. A properly designed PMP section would likely provide adequate treatment for an expanded source area. Combined with data from the literature, it is recommended that permeable pavements, in general, be given strong consideration for effective source treatment of urban storm-water runoff.

The hydraulic performance of drip-irrigation emitters is very important for the design of drip-irrigation systems, especially when commonly available high-head emitters are used in low-head drip-irrigation systems. This study tested three representative commercially available emitters using a custom-made constant-head setup to assess the hydraulic performance in working heads ranging from 0.2 to 10 m. The coefficient of manufacturing variation ($C_v$) and the emitter discharge exponent ($x$) were calculated. All the tested emitters were found suitable for low-head drip irrigation systems, but there was a clear effect on the coefficient of manufacturing variation and the emitter discharge exponent of the tested emitters at working heads of less than 1.0 m.

The channel dimensions leading to the critical state of flow are important in the design of channel transition. This work describes a methodology for determining the channel dimensions leading to the critical state of flow to maintain given specific energy or specific force conditions. The developed methodology works on extracting a two equal roots/factors of the specific energy and specific force expressions, where the two equal roots/factors yield the critical flow depth. The methodology is illustrated for trapezoidal, rectangular, triangular, and parabolic channels.