| South China karst area,though rich in rainfall,is commonly stricken with a high incidence of drought,affected by its special karst geological features.And still,soil and water loss in this area has its particularity,presenting the dual nature of loss from both the surface and the underground.The low soil formation rate has given rise to extremely serious soil and water loss here.Accordingly,no vegetation or soil cover on the surface creates the rocky desertification landscapes.The key problem of rocky desertification control is to control soil erosion,of which water is the most important factor.Meanwhile,agroforestry in karst areas,as a water-saving value-added industry,meets the needs of developing ecological derivative industry to control rocky desertification.Of this system,the study on the occurrence and transformation mechanism of"five water"and its efficient utilization can reveal the law of rational allocation of agricultural forestry in accordance with land and time,which provides theoretical basis for the efficient utilization model of water resource.This study is based on the water-saving technology of agroforestry allocation and that of cultivation,guided by the interdisciplinary theory of efficiently using water resources.Between2016 and 2020,field trials and monitoring were conducted to study the three typical karst research areas:Bijie Salaxi,Guanling-Zhenfeng Huajiang and Shibing karst research area,which represent different geomorphologic structures and rocky desertification environments in southern karst regions.In all,35 runoff rainfalls were monitored in 15 runoff plots;a total of 1810 soil samples were collected from 26agronomic water-saving plots and 18 engineering water-saving plots,with their physical properties analyzed in the laboratory.The study also monitored soil evaporation 1080 times,had 21 plant immersion tests,and tested the transpiration rate of 21 crops 592 times.Combined with the weather data,using statistical analysis and mathematical model building,this study explored the occurrence-and-conversion mechanisms and laws of rainfall,surface water,soil moisture,groundwater and biowater of agroforestry.Then,on the bases of the revealed mechanisms and laws,this study further constructed the suitable models,developed the corresponding technologies,further demonstrating,verifying and promoting them.The ultimate aim is to provide theoretical technological reference to the efficient utilization of water resources and the development of ecological industry in the national desertification control.(1)The law and the mechanism of the occurrence and conversion of"five water"are discussed under different grades of rocky desertification;the characteristics of efficiently utilizing water resources through agroforestry are expounded in the study;it is revealed that there is variation in the occurrence effect of water resources by agroforestry under different rocky desertification environments;it is also revealed that temperature,biomass,soil hydraulic characteristic parameters have an effect on the occurrence and transformation of"five water".There is a coupling relationship among the distribution of available precipitation and rainfall,land evaporation and soil evaporation under different rocky desertification degrees.Of them,the available precipitation has the lowest distribution in the medium-intensity rocky desertification area,while the soil evaporation and land surface evaporation shows the highest distribution in the medium-intensity rocky desertification area.To different degrees,agroforestry has the ecological benefits of reducing surface runoff,reducing transpiration rate and inhibiting soil evaporation.As for its inhibition and control of surface runoff,its inhibition of soil water evaporation,its increase of groundwater occurrence,and its reduction of transpiration rate of agroforestry,the results are shown as follows.The best ecological benefit occurs in the potential-mild rocky desertification environment.The ultimate benefit of water resources occurrence presents as latent-mild rocky desertification>non-potential rocky desertification>medium-intensity rocky desertification.In the"five water"transformation,the contributions of surface water,groundwater,biowater and soil water,to precipitation,are respectively 0.14-12.71%,9.43-30.20%,9.79-49.97%and40.72-82.58%.The comparative study shows that agroforestry system in the potential-medium rocky desertification environment has the highest water resources retention benefit,improving the water use efficiency.Drought stress contributes to the improvement of water use efficiency.The medium-intensity rocky desertification environment is affected by drought stress,which leads to the highest water use efficiency.Drought stress,temperature,soil hydraulic characteristics,biomass and other natural factors comprehensively affect the occurrence and transformation of"five water"resources,showing certain regularity and variation.Obviously,knowing the regularity and difference is beneficial to further reveal the water-saving and water-retaining mechanism of agroforestry and provides theoretical support for the development of water-saving value-added ecological derivative industries.(2)This paper discusses the law of occurrence and transformation of water resources and efficient utilization of water resources in agroforestry under agronomic and engineering water-saving strategies;it reveals the characteristics of soil water occurrence and transformation,as well as the characteristics of plant water inhibition and evaporation under different measures;it shows the mechanism of evaporation inhibition and evaporation reduction under different water-saving measures.Straw mulching increases the soil surface fertility,while the mechanism of water diversion by fertilizer improves the surface soil moisture content;the soil moisture content is relatively low in the middle layer,indicating that the roots of crops are mainly distributed in the 10-20cm soil layer.Compared with the control group,the measures for the agroforestry,straw mulching+water retaining agent,straw mulching,water retaining agent and plastic film mulching,reduce the evaporation of soil water,increase the soil water content,and improve the water use and water resource retention efficiency.Evidently,the composite measures,when compared with the single measures,can improve the water resource occurrence benefit and water use efficiency.Under drought stress,the water use efficiency is still the highest in the medium-intensity rocky desertification areas with water-saving measures.The arrangement of agronomic measures and engineering measures inhibit soil evaporation,increase soil moisture content,reduce the conversion rate of soil water to atmospheric water and the transpiration rate of agroforestry,and improve the water use efficiency and then the retention benefit of water resources.Through the measures of water-saving and water conservation,the agroforestry system reduces the dissipation of water resources,reveals the evapo-transpiration inhibition and reduction effects of agroforestry and the efficient utilization of water resources based on the retention and transformation of"five water",and proves the feasibility of the high-efficient utilization of water resources by adopting water-saving and water conservation measures in the agricultural forestry system in karst areas.(3)By applying the transformation law of“five water”,combining with the water saving and occurrence mechanism of agroforestry,this study constructs the agroforestry-based models of efficient utilization of water resources under different rocky desertification environments in Bijie,Huajiang and Shibing,and develops the common key technologies.It still integrated the technological systems(no-potential rocky desertification,potential-mild desertification,and medium-intensity rocky desertification)of efficiently utilizing water resources.Through the employment of the strategies to save water and efficiently utilize water resources in the agricultural forestry,three models are formed:the model of highly-efficient water resources occurrence and value-added water saving in agroforestry in Bijie Salaxi karst plateau mountainous areas with potential-slight rocky desertification;the model of effectively surface-ground water transformation and value-maintenance water saving in agroforestry in Guanling-Zhenfeng-Huajiang karst plateau valleys with medium-intensity rocky desertification;the model of highly efficient occurrence of the soil-biological water and value-added water saving in agroforestry in Shibing karst mountain valleys with the non-potential rocky desertificationt.Briefly,they are referred to as“Bijie model”,“Huajiang model”,and“Shibing model”.The technologies existing the models are summarized and extracted.The common key technologies and technical systems are researched and developed,including agroforestry configuration,plastic film cover,roof rainwater collection,surface-groundwater joint dispatching,slope rainwater collection,ecological pool,water-saving irrigation,dwarf dense planting,undergrowth cultivation and grass cover.For non-potential,potential-mild,moderate-intensity rocky desertification environments,different technologies are integrated,of which are fertilizer coupling,uncultivated grass mulch to conserve moisture,bottle root irrigation,hardened pavement rainwater harvesting,roof rainwater harvesting,and surface groundwater combined scheduling.(4)It shows in this study that the water resources saving and efficient utilization model of agroforestry are scientific and operable,with favorable results of application and demonstration.This assures the suitability to promote these models across other karst areas of the 8 southern provinces in China.The areas,most suitable for Bijie model,Guanling-Zhenfeng model and Shibing,account for 37.12%,20.52%and38.38%of the total area of the eight provinces,respectively.Since 2016,the research team has researched and investigated the current utilization situation of agroforestry and water resources in Bijie Salaxi,Huajiang and Shibing.On this basis,combined with the demonstration and research results in the previous investigation,over 6000hm~2 from the three research areas are selected to demonstrate the water-saving and efficient utilization of water resources of agroforestry.This practice encourages the local residents to develop ecological industries,manifesting favorable ecological,economic and social benefits.It is confirmed that the development of water-saving value-added agroforestry is conducive to the restoration of the degraded rocky desertification environment,the control of soil erosion,the promotion of vegetation restoration and the promotion of economic development.To evaluate the suitability of the models,this study,by using GIS spatial analysis and evaluation of indicators,establishes the evaluation index system of rainfall,temperature,altitude,landform type,lithology,slope,soil thickness,soil erosion intensity,soil type,population density,and per capital GDP.The results show that the most suitable,more suitable,basically suitable,barely suitable and inappropriate promotion areas are as follows respectively:Bijie model accounts for74.33×10~4km~2,225.03×10~4km~2,37.68×10~4km~2,52.05×10~4km~2 and 4.60×10~4km~2;Huajiang model for 39.74×10~4km~2,14.52×10~4km~2,21.90×10~4km~2,20.83×10~4km~2 and96.70×10~4km~2;and Shibing model for 74.33×10~4km~2,25.03×10~4km~2,37.68×10~4km~2,52.05×10~4km~2,4.60×10~4km~2. |
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