Journal of Irrigation and Drainage Engineering

Past research on evapotranspiration has provided sound theoretical knowledge and practical applications that have been validated through field measurements. Many different approaches have been used; however, when primary concepts and standard definitions are accepted, it is possible to find reasonable agreement among methods. This paper reviews such approaches, from Penman to Penman-Monteith. The standard concepts of potential evaporation (EP) and equilibrium evaporation (E_e), and the introduction of the climatic resistance (re), provide a better understanding of the role of the climate together with surface and aerodynamic resistances (r_s and ra). Therefore, the concept of reference evapotranspiration (ET_o), particularly the new one adopted by the Food and Agricultural Organization of the United Nations, can be better understood, as well as its limitations. Crop evapotranspiration (ET_c) is related to both ET_o and Ee. Crop coefficients (K_c) can be shown to have two components, α_o and α_c, with Kc = α_oα_c. The α_o is a function of the climatic resistance and of the aerodynamic resistances of the crop and of the reference crop. The α_c is a function of both surface and aerodynamic resistances of the crop and of the reference crop. From this analysis some ideas on future developments result that are directed toward providing compatibility between the one- and two-step calculation of ET_c.

Nitrate leaching from corn production systems and the subsequent contamination of ground and surface waters is a major environmental problem. In field plots 75 m long by 15 m wide, the writers tested the hypothesis that subirrigation and intercropping will reduce leaching losses from cultivated corn and minimize water pollution. Nitrate leaching under subirrigation at a depth of either 0.7 m or 0.8 m below the soil surface was compared with leaching under free drainage. The cropping systems investigated were corn (Zea mays L.) monoculture and corn intercropped with annual Italian ryegrass (Lolium multiflorum Lam. cv. Barmultra). The effects of three fertilizer application rates (0, 180, and 270 kg N ha⁻¹) on leaching were investigated in the freely drained plots. The greatest annual loss of ${\mathrm{NO}}_{\mathit{3}}^{\mathit{-}}-N$ in tile drainage water (21.9 kg N ha⁻¹) occurred in freely draining, monocropped plots fertilized with 270 kg N ha⁻¹. Monocropped plots fertilized with 270 kg N ha⁻¹, with subirrigation at 0.7 m depth, resulted in annual nitrate losses into tile drainage of 6.6 kg N ha⁻¹, 70% less than under free drainage. Annual soil denitrification rates (60 kg N ha⁻¹) with subirrigation at 0.7 m were about three-fold greater than under free drainage. Intercropping under free drainage resulted in a 50% reduction in tile drainage loss of ${\mathrm{NO}}_{\mathit{3}}^{\mathit{-}}-N$ compared with monocropping. Off-season (November 1, 1993, to May 31, 1994) tile drainage losses of ${\mathrm{NO}}_{\mathit{3}}^{\mathit{-}}-N$ (7.8 kg N ha⁻¹) from freely draining monocropped plots accounted for 30% of the annual tile drainage losses.

Subsurface drainage in irrigated agriculture presents an interesting challenge in that rapid drainage is required after an irrigation or rainfall event to alleviate any waterlogging, but during irrigation events drainage is undesirable. High rates of drainage during an irrigation event will lead to poor application efficiency. Also, when using mole drains, which rely entirely upon soil cohesion to retain their structure, high rates of water flow can lead to their rapid collapse. This paper compares various methods of reducing rates of irrigation water flow into mole drains and the subsequent effect on mole stability. Localized crack sealing methods using a trailing wedge behind the mole plough, followed by tractor wheeling or rotovation, were unsuccessful. However, generalized surface tillage to create a fine surface tilth about 100 mm deep was successful in reducing mole flow rates and improving stability. Preventing discharge from mole drains during irrigation was particularly effective in reducing mole flows and was not found to reduce mole stability compared to high rates of drainage throughout an irrigation. Sprinkler irrigation was found to result in lower mole flow rates than flood irrigation, and improved mole stability. Positioning the mole away from areas where water is ponded on the soil surface for irrigation, such as furrows, also resulted in lower flows and improved stability.

Working on the assumption that temperature affects water viscosity and emitter geometry, the writers have developed a dimensional analysis approach to the potential pressure-discharge relationship to estimate discharge sensitivity to temperature. Its accuracy was validated by experimental data. Flow rates were measured under controlled conditions on samples of six commercial emitters. Pressures of 100, 150, and 200 kPa were applied. Water temperature was modified alternating heating-cooling temperatures from 20 to 40°C. A factorial analysis of variance was performed on the observed data with the results presented as coefficients of variation. Emitter discharge was found to be insensitive to the alteration of heating-cooling temperatures. Discharge of the helical long-path emitter increased with increasing temperature at a maximum rate of 0.7%/°C. In contrast, it decreased in the vortex emitter at a maximum rate of 0.4%/°C. The dependence of orifice-type emitter discharge on temperature was less significant. Discharge of compensating emitters was affected as much by the operating time of each test as by the time elapsed between consecutive tests, and was not dependent on temperature change.

The Laplace transform of convolution equation, which relates aquifer response to boundary perturbation, expresses explicitly the hydraulic diffusivity in terms of the Laplace transform parameter, changes in stream stage, and fluctuations of piezometric level at a point near the stream. Hydraulic diffusivity has been estimated using the Laplace transform approach. The diffusivity has also been determined from observed response of an aquifer and the boundary perturbation using the Marquardt method, a least-squares optimization technique. If the observed data are free from random error, the diffusivity can be estimated accurately using the Laplace transform approach. Unlike the least-squares optimization method, the Laplace transform technique automatically gives less weight to the latter part of the aquifer response and thereby to the random error contained in it. Discrete kernel coefficients have been derived, using which the water level rise in an aquifer that can be predicted for any variation in stream stage.

In this paper a method of obtaining the effective hydraulic conductivity for partially saturated porous media is proposed. The governing moisture transport equation was modified by introducing a velocity term to make it analogous with the longitudinal dispersion equation. This equation was solved using a transformed coordinate system where the origin is moving at the mean velocity. The effective hydraulic conductivity was derived by an approach similar to the derivation of the longitudinal dispersion coefficient subject to certain limitations. The validation of the proposed approach was tested using a randomly generated set of local soil water data.

This work demonstrates the applicability of more generalized sluice gate equations to the design of baffle sluice modules. The generalized equations allow the module to be designed for any upstream head. These equations also offer additional understanding of the behavior of the baffle sluice module and demonstrate that the first baffle of the module is largely redundant. The constraint of having the minimum water level coincide with the crest of the first baffle has been removed. Through an optimization procedure, an alternative design method is suggested that will theoretically improve the performance of the baffle sluice module. Through this design technique, a module can be designed with any practical number of baffles. The effect of combined orifice-weir flow and width of the module is also discussed.