Research On Cultivated Land Productivity Based On NPP Index Corrected By Topography

On January 9,2017,opinions of the CPC Central Committee and the State Council on strengthening farmland protection and improving the balance of occupation and compensation were issued.The new requirements of Three-in-one protection of cultivated land quantity,quality and ecology are put forward.The quality protection of cultivated land is strictly required,the core of which is to ensure that the total production capacity of cultivated land will not be reduced in the future and food security will be guaranteed.The opinions is also required to"establish and improve the evaluation system of cultivated land quality and productivity,and improve the evaluation index system and evaluation method".In this context,it is necessary to carry out cultivated land productivity evaluation survey.The existing cultivated land productivity evaluation system is the cultivated land grading regulations issued in 2012 and Specification for cultivated land quality survey and monitoring conducted in 2018.Both of them are based on the national unified standard of county-level light and temperature production potential,through field sampling to test soil and hydrological data,through field survey and questionnaire analysis to obtain social and economic indicators,so as to gradually modify the light and temperature production potential to obtain the final cultivated land productivity.The two methods are time-consuming,labor-consuming and prone to errors.Therefore,this paper aims to build a new set of cultivated land productivity evaluation method,using remote sensing data to achieve the dynamic evaluation of cultivated land productivity.NPP(net primary productivity)of vegetation is the part of carbon absorbed by organisms through photosynthesis in a unit time,except the remaining part of carbon loss of plant respiration,which reflects the production capacity of plant community in a certain environment.After setting the object as cultivated land,its connotation is consistent with cultivated land productivity.Therefore,this paper chooses NPP estimation method to calculate cultivated land productivity.In order to realize the dynamic measurement,the CASA model in NPP estimation mode is selected.Only remote sensing data,meteorological data and DEM data are needed to calculate the real-time NPP.In addition to the temperature data,the most important thing is to use the slope,aspect and altitude to refine the calculation of the solar incidence angle,terrain obscuration and direct transmittance in the solar radiation,so as to realize the calculation of solar radiation in complex terrain.On this basis,Binyang County,Guangxi Province,which has a large topographic difference,is selected as the research area.Based on the landsat8 data of four periods of crop growth in March,May,October 2017 and February 2018 with typical characteristics,combined with meteorological data,DEM,land use status data and annual update database data of cultivated land quality,the following results are obtained:(1)NPP calculation resultsThe average NPP of Binyang County is 837.21,and the results sho W??that NPP of paddy field is larger than dry field and NPP in plain area is higher than hilly area.There are regional low values along the Northern Railway,the northern Xinwei town and the border of Daqiao town in the plain area.The differences of the four periods of the index are basically consistent with the growth characteristics of crops in Binyang County.Compared with the difference of NPP index before and after terrain modification,the correlation between NPP index distribution in plain area and FPAR and is enhanced,while that between FPAR and in hilly area is weakened,that is to say,the correlation between NPP and terrain factors is strengthened.(2)Verification of NPP characterizing cultivated land productivityNPP index fitted economic effects,the R~2 of paddy field was nearly 0.86,and that of dry field was 0.88.The correlation between NPP and cultivated land economy is good,and it can be replaced either in concept or in actual value.However,through GWR,it is found that R~2 in some areas is still low.In order to analyze the causes of the differences,this paper analyzes the correlation between NPP and the factors affecting the productivity of cultivated land.It is found that response sensitivity of NPP is higher than cultivated land in terms of soil properties and cultivated land topography,but not in terms of farming conditions,especially in terms of cultivated land construction after irrigation and drainage.Through conceptual analysis,it is found that NPP does not reflect the relationship between cultivated land construction and cultivated land economy.Therefore,in order to approach the concept of economy,NPP is further modified based on cultivated land construction,so that R~2 between paddy field NPP and cultivated land economy is 0.88,and that of dry field NPP is 0.89.On this basis,combined with the regional differences of the influencing factors,the analysis of poor local R~2 region shows that NPP is more meticulous and diverse in light temperature and water production potential.Secondly,the response sensitivity of NPP in soil properties and cultivated land topography was higher than that in cultivated land economy.Finally,in terms of farming conditions,through the establishment of regression equation between various influencing factors and cultivated land economy,the coefficient of farming conditions is far higher than other factors,and the influence of irrigation and drainage conditions is overstated in economy.(3)NPP applied to farmland protectionIn addition to the dynamic prediction of cultivated land productivity,NPP can be applied to the cultivated land protection in providing the data basis and characteristics to the new round of land spatial planning,especially the arrangement of comprehensive land improvement projects.It mainly involves three aspects:first,the delimitation of basic farmland protection area;second,the benefit evaluation of the farmland improvement projects,The third is the zoning and arrangement of farmland improvement projects in low and medium yield fields.In fact,in the aspect of NPP application in farmland regulation benefit evaluation,four plots of Heji town were selected for comparison of cultivated land before and after the improvement.It was found that the productivity of cultivated land in three plots had been effectively improved,but the benefit of land remediation in karst landform area was not obvious.In the delimitation of basic farmland,the first 20%of NPP index can be evaluated as high yield(23063.42 ha.).Because its area is larger than the target of basic farmland(14046.67 ha.)set in the land remediation plan,further cluster analysis is carried out to divide it into two categories.The cultivated land with high productivity,concentrated contiguity and high accessibility can be directly classified as basic farmland protection area(16321.64 ha.),which is slightly higher than the planning goal as a whole,but some of the town goals are not achieved.On this basis,taking Xinqiao Town as an example(the remaining target is 529.29 hectares),the cultivated land with high productivity but lack of centralized connectivity and accessibility is divided into the basic farmland construction area(1322.61 ha.),and its indicator characteristics are divided into three types and arrange time-series,i.e.traffic facility construction,farmland leveling construction and comprehensive construction,which are classified into the basic farmland protection area or supplementary planning Repository.In the planning of low and medium yield field improvement,the plot with NPP index in the latter 20%is classified as low and medium yield field(9327.67 Ha.).Because the low and medium yield field is mainly distributed in the area with special geographical environment,the general soil consolidation effect is poor and easy to cause ecological damage.The three factors FPAR,and solar radiation in CASA model are used as grouping criteria to classify the characteristics of low and medium yield field.In this condition,the investment in science and technology with high cost has a direction and focus.It is divided into soil construction,irrigation and drainage construction,comprehensive construction and ecological protection type,increased investment in scientific and technological transformation,and implemented ecological friendly remediation,while ecological protection type mainly focuses on ecological restoration and does not carry out cultivated land production.In conclusion,this paper calculates the NPP of Binyang County by using the CASA model improved by terrain,which has a good correlation with the annual renewal results of the existing cultivated land quality,and has advantages in light and temperature production potential,soil conditions and terrain conditions.NPP can be used to represent the cultivated land productivity of Binyang County.The calculation results are applied to the delimitation of basic farmland,the regulation construction planning of medium and low yield farmland and the benefit evaluation with high efficiency.Of course,this paper is not perfect enough,which needs to be further studied on the differences of edible parts of different crops,the relationship between cultivated land use conditions and carbon storage,and the remote sensing data deviation of dry land and paddy field.