| The technology of film mulched drip irrigation has been widely applied in cotton field of Xinjiang region, because of its significant advantages on saving water, decreasing soil salinity in root zone, and consequently increasing cotton yield. Under the condition of arid climate and limited irrigation amount, researchers and government in recent years were pay close attention to the trend of soil salt accumulation effect in root zone of cotton field under film mulched drip irrigation during a long-term applied.The object of this study were two typical planting modes of drip-irrigated cotton under film in the Manas river basin of Xinjiang, and the field experiments were conducted over two seasons (Exp.1 and 2 in 2011 and 2012, respectively) for two planting modes (traditional mode and super wide film mulch mode). Based on analyzed and generalized the root distribution characteristics of cotton under different planting modes, the two-dimensional root-water uptake model and soil water and salt transport model were established. After verified, the models were used to simulate the movements and evolution trends of soil water and salt during long-term applied for the technology of film mulched drip irrigation in cotton fields under different types of hydrological years condition. The main conclusions of this study were as follows:1. Two-dimensional root distribution characteristics of cotton under film mulched drip irrigation of two typical planting modes were shown:in general, root length density (RLD) decreased with depth, but increased with time as the plant grew. Most roots were concentrated within the upper soil layer (above 0-50 cm). Moreover, since the two narrow cotton rows were irrigated or controlled by one line of drip tape, the high-frequency drip irrigation would basically meet the water requirement of cotton, and stimulated lateral growth of roots. The lateral growth and the corresponding interaction of roots between neighboring rows would be expected to gradually diminish the effect of the small spatial difference between wide and narrow rows regarding root growth along the horizontal direction.2. To build a reliable root-water uptake model, the concept of normalized root length density (NRLD) distribution was introduced to generalize the root distribution of cotton under film mulched drip irrigation, and the statistics results indicated that:(1) In relative depth(zr) direction (0=≤zr≤1), the NRLD distribution of cotton along the vertical zr-direction decreasing with depth, the maximum and minimum value of NRLD at (near) the soil surface (zr=0) and root zone bottom (zr=1), respectively; The conventional statistical characteristic values were used to evaluate the difference of NRLD in relative horizontal (xr) direction, and it was indicated that measured NRLDs were similar with the average (MN) values NRLD from five different horizontal sampling locations at the same relative depth (zr) for the different treatments at different growth stages; and corresponding to standard deviation (SD) of the NRLD from five different horizontal sampling locations were very low at the same relative depth (zr) for the different treatments at different growth stages, and coefficient of variation (CV) of the NRLD from the upper soil layers were small since most roots were concentrated within there. Therefore, the differences of NRLD along the horizontal xr-direction were determined to not be significant, and the NRLD distribution in vertical zr-direction could be used to reflect the main characteristics of the root distribution for cotton and could be generalized by a vertical one-dimensional function under film mulched drip irrigation.(2) The vertical one-dimensional NRLD function that α(1-zr)α-1 could be used to illustrate the distribution of NRLD, and the processes of optimization fitted and verified as follows:firstly, the measured NRLD data of 2012 were used to fit the function, then NRLD data of 2011 was used to verify, and the fitted results were good. Next, the measured NRLD data of 2011 and 2012 (including different planting modes, fertigation schedules and growing stages, etc) were used to refit the function and the fitted coefficient a (characterizing the maximum value of NRLD at the soil surface) was 1.96. The results shown the NRLD function with a=1.96 could be superior for describing the NRLD distribution of cotton under film mulched drip irrigation. Finally, all available 2314 data of NRLD distribution of cotton were collected from literature to verify the generalized NRLD function. The generalized NRLD function for cotton in this study maybe applied under a wide range conditions in the cotton field.3. The soil water flow and solute transport under film mulched drip irrigation in the cotton field were usually generalized as a longitudinal section two-dimensional distribution. The vertical one-dimensional NRLD function could be used to develop a model of root water uptake, in order to avoid the tedious processes for root sampling and measured in the cotton field and to facilitate simulation the dynamics distribution of soil water and salinity at different growing stages continuously. Based on the built models of root-water uptake, soil water and salinity in this study, the soil water and salinity distributions in the cotton field were simulated by the models at growing stages of 2011 and 2012, and the simulated and measured soil water and salinity distributions were also in good agreement. The modeling results all indicated that soil physics parameters and the developed models of root-water uptake, soil water and salinity in this study were reasonable, and could be used to simulate the soil water and salinity dynamics distributions under film mulched drip irrigation in cotton field.4. Under the condition of irrigation schedules and normal cultivation practice experience in cotton field under film mulched drip irrigation and the local representative different hydrologic years (25%, 50%,75% and the years of 2002～2011) of Shihezi region in Xinjiang, the verified soil water and salt transport model was used to model the movements and evolution trends of soil water and salt during long-term (10-year) in film mulched drip-irrigated cotton field. The simulated results were shown:(1) There were also calculated the intra-annual and inter-annual soil salt banlances of cotton field under the representative different hydrologic years during 10-year applied the film mulched drip irrigation technology continuously with higher soil salinity profile. Although soil salinity of 0-30 cm and 0-60 cm in root zone were fluctuated more severely, and the processes of salt deposition and desalination appear alternately; generally, the annual changes of soil salinity of 0～100 cm in root zone were all illustrated similar trends and phenomenon of salt desalination. Since the annual drainage flux of the bottom in the root zone were small and shown a decreased trends, there have gradually decreased trends for soil salt desalination at the 0～100 cm in root zone.(2) Under the condition of irrigation schedules and 25% hydrologic year, during 10-year applied the film mulched drip irrigation technology continuously at the same site with lower soil salinity profile, the simulation results shown that the soil salinity of 0～100 cm in root zone accumulated year by year, and soil salt were severely accumulated in the 40～80 cm of root zone, especially. During a long-term cultivation and cropping, the accumulated salt in the root zone may have an adverse effects for cotton growth and the sustainable development of agriculture in Xinjiang region. |
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