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991.
992.
A.J. Clemmens 《Irrigation and Drainage Systems》1998,12(2):85-113
This paper describes a new procedure for the design of level-basin irrigation systems which is based on the irrigator's cutoff criteria. Design with this procedure, hopefully, will be more transferable to users and will allow basin designs to be adopted to local practices. Comparisons are made to existing design procedures, and it is shown that many of them can be expressed in terms of this new procedure, available in the form of a computer program. A sensitivity analysis was used to confirm the desirability of these new guidelines on design. The sensitivity analysis also showed that the cutoff distance can be adjusted to account for changes in vegetative roughness. 相似文献
993.
水泵叶片空间曲面计算和数控加工 总被引:4,自引:0,他引:4
论述了水泵叶片空间曲线的计算和用数控铣床加工的基本方法,并介绍在江苏省江都第一抽水站更新改造工程中成功应用的实例,现场实测结果表明数控加工的叶片能够保证水泵设计性能。 相似文献
994.
995.
Quality of agricultural soils is largely a function of soil organic matter. Tillage and crop management impact soil organic matter dynamics by modification of the soil environment and quantity and quality of C input. We investigated changes in pools and fluxes of soil organic C (SOC) during the ninth and tenth year of cropping with various intensities under conventional disk-and-bed tillage (CT) and no tillage (NT). Soil organic C to a depth of 0.2 m increased with cropping intensity as a result of greater C input and was 10% to 30% greater under NT than under CT. Sequestration of crop-derived C input into SOC was 22±2% under NT and 9±4% under CT (mean of cropping intensities ± standard deviation of cropping systems). Greater sequestration of SOC under NT was due to a lower rate of in situ soil CO2 evolution than under CT (0.22±0.03 vs. 0.27±0.06 g CO2–C g−1 SOC yr−1). Despite a similar labile pool of SOC under NT than under CT (1.1±0.1 vs. 1.0±0.1 g mineralizable C kg−1 SOC d−1), the ratio of in situ to potential CO2 evolution was less under NT (0.56±0.03) than under CT (0.73±0.08), suggesting strong environmental controls on SOC turnover, such as temperature, moisture, and residue placement. Both increased C sequestration and a greater labile SOC pool were achieved in this low-SOC soil using NT and high-intensity cropping. 相似文献
996.
Phosphorus losses from arable land in England 总被引:7,自引:0,他引:7
J. A. Catt K. R. Howse R. Farina D. Brockie A. Todd B. J. Chambers R. Hodgkinson G. L. Harris J. N. Quinton 《Soil Use and Management》1998,14(S4):168-174
Abstract. Concentrations and annual loadings of molyhdate reactive P (MRP) and total (including particulate) P (TP) are reported from field drainage, catchment and erosion experiments in England. Annual losses through field drains and in catchment runoff were 0.037-0.74 kg MRP/ha and 0.37-2.64 kg TP/ha, but those in surface runoff from experimental plots measuring erosion were generally much greater (often > 3 kg MRP/ha and up to 32 kg TP/ha in a wet year). Amounts of TP in drainflow and catchment runoff depended upon factors influencing soil dispersibility, such as particle size distribution and calcium carbonate content. The results to date suggest that P losses in surface runoff and erosion from arable fields to water are best limited by: (a) maximizing crop cover, using minimal cultivation practices and where possible planting crop rows across rather than up and down the slope, (b) avoiding cultivation practices that result in dispersion of soil particles, and (c) avoiding application of P fertilizer to wet soils when rainfall is likely soon after application. Consideration should he given to maintaining field drains below peak efficiency to reduce subsurface P losses. 相似文献
997.
宇宙线散落核素7Be具稳定的物质来源和53.3天的半衰期,提供了示踪环境微粒季节性迁移的必要条件;其在土粒中主要存在于Fe-Mn氧化物及有机质结合相,适应于季节性微粒迁移示踪;它在表土层中的垂直分布特征提供了示踪微粒季节性迁移的充分条件。7Be在表土层中的累计值、表观比活度、最大渗透深度及相对侵蚀(或堆积)速率是土粒迁移的示踪参数。 相似文献
998.
坡地土壤水分动态及耗水规律研究 总被引:14,自引:2,他引:12
8年长期坡耕地土壤水分研究表明:裸地2m土层水分季节性变化呈倒S型;40~100cm内土层水分对作物供水极为重要;在降水量为500mm左右的黄土丘陵沟壑区,降水能完全满足坡耕地作物生长发育;在降雨因素不成为限制作物生长发育主要因素时,水平沟耕作可以有效地控制地表蒸发,降低作物耗水量;人工草地耗水强度趋势为:沙打旺>草木樨>紫花苜蓿>红豆草>柠条(二年生);3年生人工草地主要利用50~200cm土层水分,8年生人工草地利用150~400cm土层水分,且8年生人工草地土壤水分恢复维艰。 相似文献
999.
1000.
湖北省几种土壤的粘土矿物组成与表面化学性质 总被引:4,自引:3,他引:1
研究了湖北省棕红壤、黄棕壤、水稻土和灰潮土的粘土矿物组成、电荷性质及对磷的吸附。结果表明:①供试土壤的粘土矿物组成都以水云母为主,高岭石和1.4nm矿物含量较少,1.4nm矿物种类不同且比例变异大;②供试土壤的负电荷量(CEC8.2)介于9.2 ̄19.1cmol/kg间,永久负电荷占CEC8.2的50% ̄64%,阴离子交换量(AEC)为0.75 ̄2.21cmol/kg,电荷零点(PZC)介于2.3 相似文献