新疆膜下滴灌棉田暗管排盐的数值模拟与分析Ⅱ:模型应用

Evaluation of salt discharge by subsurface pipes in the cotton field with film mulched drip irrigation in Xinjiang, China Ⅱ: Application of the calibrated models and parameters

为了降低新疆地区盐碱棉田根区土壤盐分含量,尤其是排走长期使用膜下滴灌技术造成的下部根系层(40~60 cm)累积盐分,针对当地实际情况设计了2种改进排盐模式,分别是淋洗防渗排盐模式(情景1)和暗管局部冲洗排盐模式(情景2),利用经过校验的模型和参数对不同模式下暗管排水、排盐动态过程进行模拟。情景1在暗管下方铺设一定宽度(L_f,分别设定为20、50、100、250和500 cm)的防渗材料以增加汇流面积,提高排水、排盐量;情景2先通过暗管直接供水湿润周围土壤,达到设定时间(T_i,分别设定为0.25~10 d的9种情形)后停止供水,然后再通过暗管进行排水、排盐,以期利用较小的冲洗定额达到排盐的目的。对情景1进行模拟时,以0~40 cm和40~60 cm土壤含盐量分别低于3 g/kg和6 g/kg作为结束淋洗的标准进行对比分析,结果表明:当暗管处于非饱和区域、下方无防渗处理(传统暗管排盐模式)时,其排盐率仅为9.8%,单方水的排盐效率约1.86 kg/m^3;在暗管下方进行防渗处理有利于增加暗管排盐量,排盐率可达11.9%~32.1%,排盐效率可提高至2.27~3.15 kg/m^3;然而,随着Lf增加,施工难度和成本均会大幅上升,尤其是当Lf>100 cm,单方水排盐效率的提高程度却较为有限,因此建议Lf在100 cm以内。而采用暗管供水进行局部冲洗(情景2)时,即使在暗管下方进行防渗处理,其单方水的排盐效率最高也不超过0.6 kg/m^3,整体表现并不理想,在新疆地区使用不太现实。

Efficiency of salt removal using subsurface pipes（SSPs） located in the unsaturated zone of agri-cultural soils is very low due to a limited effective flow collecting area（Scf）. In this study, the dynamicsof soil water movement and salt transport were simulated using the calibrated models and parameters forHYDRUS-2D/3D in order to investigate two possible methods for improvement of ameliorating saline soilsin cases where drainage pipes are located well above the groundwater table. The first improvement methodattempted to enlarge Scfby laying seepage-proof material（SPM） underneath the SSPs. The effect of SPMwidth（Lf） and distance between SSPs was evaluated. Simulations continued until the salinity of the 0~40and 40~60 cm soil layers was less than 3.0 and 6.0 g/kg, respectively, levels believed to allow successfulcotton cultivation. Compared to the treatment without SPM, the salt discharge ratio（SDR） and the wateruse efficiency for salt discharge（WUESD） were enhanced from 11.9% and 1.86 kg/m3 to a maximum of32.1% and 3.15 kg/m3, respectively, when a SPM was present. While, in general, increasing LfenhancedWUESD,considerations of installation costs suggest an optimal practical Lfof between 20 and 100 cm. TheSDR and WUESD were further enhanced by decreasing the space interval between pipes from 500 cm,typi-cally found in commercial fields, to 200 cm. In additional simulations, the SSP was used to both supplywater and discharge leachate, eliminating the need for wetting of the entire profile and potentially enhanc-ing salt discharge efficiency. Results showed that the approach had limited practical value as very little saltcould be discharged through the SSP,and the maximum WUESD was not more than 0.6 kg/m3 even whena SPM was laid beneath SSP and the space interval between pipes was narrowed to 200 cm. The modelingapproach presented should be useful in evaluation of further approaches using SSP to improve reclamationof saline agricultural soils where drainage of saturated zones is unpractical.