THE PLANT INDICATOR METHOD(PIM)FOR MAPPING CROP YIELDS IN RESPONSE TO DROUGHT,SOIL SALINITY,AND OTHER STRESSES

摘要

The general purpose Plant Indicator Method(PIM)of mapping plant stresses and crop yield is described using soil salinity as an example.Three equations that interrelate spectral observations expressed as vegetation indices(VI) and leaf area index(LAI),absorbed photosynthetically active radiation(APAR),crop yield(Y)and root zone soil salinity validate the approach.Vegetation indices are useful because they measure the photosynthetic size of plant canopies.For our application,soil salinity was measured as electrical conductivity of saturated soil extracts(ECe) (or electromagnetic(EM)induction observations calibrated to ECe)and crop yield(Y)was measured manually(or with yield monitors)at sample sites in representative fields.Digital counts for the same sites were extracted from the satellite data for cropped fields already entered into a geographic information system(GIS).Regression equations were developed that estimate root zone weighted ECe(WEC)and crop yields,respectively,from the satellite observations.Then every pixel from the cropped fields,crop by crop,is submitted to the regression equations,the estimates are sorted into yield(or salinity)intervals,the number of pixels in each category is cumulated,and each category is assigned a color.Both this supervised classification and the unsupervised one,described in the text, provide tabular summaries and maps that quantify the extent of soil salinity and its effects on yield.The yield losses summed across salinity classes,times current market prices provide the economic loss,to society and farmers, needed for cost:benefit analyses of proposed reclamation and amelioration efforts.The PIM was elected for operational use in Mexico,and has been applied to 362,000 hectares of wheat,cotton,corn,and grain sorghum in four irrigation districts to date.It is readily adaptable to assessment of drought,freeze or hail damage,foliar or root diseases,improvements in crop yield in response to reclamation,insect infestations,weed and shrub encroachment, etc.provided their signature is sufficiently distinctive.