Influencing Factors and Simplified Model of Film Hole Irrigation
Film hole irrigation is an advanced low-cost and high-efficiency irrigation method, which can improve water conservation and water use efficiency. Given its various advantages and potential applications, we conducted a laboratory study to investigate the effects of soil texture, bulk density, initial soil moisture, irrigation depth, opening ratio (ρ), film hole diameter (D), and spacing on cumulative infiltration using SWMS-2D. We then proposed a simplified model based on the Kostiakov model
... Kostiakov model for infiltration estimation. Error analyses indicated SWMS-2D to be suitable for infiltration simulation of film hole irrigation. Additional SWMS-2D-based investigations indicated that, for a certain soil, initial soil moisture and irrigation depth had the weakest effects on cumulative infiltration, whereas ρ and D had the strongest effects on cumulative infiltration. A simplified model with ρ and D was further established, and its use was then expanded to different soils. Verification based on seven soil types indicated that the established simplified double-factor model effectively estimates cumulative infiltration for film hole irrigation, with a small mean average error of 0.141-2.299 mm, a root mean square error of 0.177-2.722 mm, a percent bias of −2.131-1.479%, and a large Nash-Sutcliffe coefficient that is close to 1.0. area covered by film mulch considerably increased to 20 million hm 2 in 2014, and its use increased fourfold (from 642 to 2580 megatons) from 1991 to 2014. Film hole irrigation has also developed along with film-mulched techniques for crops with wide line spacing  . Film hole irrigation is a relatively new irrigation method that involves completely covering a bordered field with plastic film with holes of uniform size  . Water penetrates into the soil through the holes during irrigation, and seedlings sprout through these holes on germination. Compared with the traditional surface irrigation method, this technique considerably reduces water losses and improves the uniformity of irrigation along the long direction of the border [6, 16, 18, 19] . Mathematical models and software, such as winSRFR  , have been widely used in the design of irrigation systems to improve the efficiency of water application and the uniformity of water distribution. Film hole irrigation is somewhat similar to point-source irrigation, in that water infiltration occurs in the region directly around the film hole. Unlike other point source irrigation systems, in which the water is transported by tubes, in film mulch irrigation, water is applied to the top of the border or furrow and it flows above the applied film mulch to the end of the border or furrow under the influence of gravity, similar to surface irrigation. In the design of a surface irrigation system, a zero-inertia model and winSRFR software are the most useful tools. In the model, the infiltration and roughness are the key parameters to be determined. As with surface irrigation, the infiltration characteristics of film hole irrigation are fundamental for determining a field film hole irrigation scheme with high application efficiency and distribution uniformity. Therefore, studying a simple and easily-estimated infiltration model of film hole irrigation is essential. Numerous laboratory studies and some field studies on film hole infiltration have been conducted in the last two decades in China; such studies have mainly focused on wetting patterns, empirical modeling, and infiltration characteristics, including single and multipoint source infiltration [18, 21, 22] . However, the models of all of these studies have been empirical descriptions of some specific soils; therefore, a more universally applicable model must be developed. Numerical simulation is often used in soil research. From a theoretical point of view, film hole irrigation involves three-dimensional point-source infiltration under a low-pressure water head (i.e., irrigation depth). Similar to subsurface drip irrigation, film hole infiltration is affected by many factors, such as soil texture, bulk density (γ d ), irrigation depth, film hole diameter (D), and hole spacing (i.e., distance between the centers of two neighboring holes). With the development of computer simulation techniques, numerical simulations based on the theory of unsaturated soil water movement are being increasingly used to study soil water infiltration. Several programs, such as HYDRUS and SWMS-2D, are often used to simulate soil water movement, and these have been effectively and accurately applied to subsurface drip irrigation       for predicting wetting patterns and infiltration characteristics, yielding more generally applicable results  . Therefore, the objectives of this study were: (1) to assess the feasibility of SWMS-2D for simulation of the cumulative infiltration of film hole irrigation through a laboratory experiment; (2) to investigate the effects of various influencing factors on cumulative infiltration in film hole irrigation and then to select the dominant factors; and (3) to propose and verify a simplified double-factor model that estimates the infiltration of film hole irrigation. Materials and Methods Laboratory Experiments Given the realities that constrain film hole irrigation in the field, the film hole diameters and spacing are generally determined by the actual situation of field crops. The diameters of film holes and the spaces between them are typically 3-8 and 12-30 cm, respectively. The opening ratio, ρ, is defined as the ratio of the area of the open holes to the total area under the plastic mulching; in general, ρ is 2-5%; the irrigation amount is 225-450 m 3 ·hm −2 ; and the irrigation depth relative to the film mulch is kept constant within a range of 4-6 cm [30, 31] . The soil is usually irrigated when the soil water content Water 2017, 9, 543 3 of 18 (SWC) is at 40-60% field capacity. Considering all of these irrigation variables, we designed eight treatments for our experiments (Table 1) .