Spatial modeling is one of the method for understanding and predicting environmental variables. soil surface moisture is a key variable for drought description, water and energy exchanges between earth and atmosphere. In addition soil moisture affects many environmental phenomena such as runoff, soil erosion, and crop production, due to the non-constant spatial and temporal conditions Environmental parameters is highly changeable. the purpose of this paper is to evaluate the overall regression model and geographically weighted regression in spatial modeling of soil moisture in Fars province. soil moisture distribution as dependent variable and precipitation, snow equivalent water, vegetation index and topographic wetness index were selected as independent variables and then, using the general regression model and geographically weighted regression is used to model the spatial modeling. based on the evaluation criteria, the results showed the GWR model has better explanatory power with the R2=0/71 and a better estimate than the overall regression model with the R2=0/66. The spatial factors of precipitation and topographic wetness had the most positive effect and evapotranspiration had a negative effect on soil moisture in the study area.
Extended abstract Introduction soil surface moisture, in addition to its contribution to the hydrological cycle, is one of the most important components of the earth 's crust because it plays a controlling role between the earth 's surface and the atmosphere as well as water, energy and carbon. soil moisture plays an important role in the global energy cycle and controls the energy conversion process. researchers have shown that there is strong feedback between the soil and the climate of the region. spatial and temporal variations of soil moisture variability differ both at the surface and below the surface. although remotely sensed images have made good estimates of soil moisture in a large area, there is no systematic global network to monitor soil moisture. estimation of soil moisture from remote sensing data due to extensive coverage can optimize surface and surface moisture conditions in different time periods. soil moisture can be estimated by using variables and soil models. identification and modeling of the variables involved in soil moisture can be a key step in the prediction of the nasal. by examining the past researches, soil moisture can be obtained depending on several factors such as climate conditions and soil moisture conditions in the past which are very diverse in different locations. the importance of changes in soil moisture in the region and its relation with other variables has not much attention to the effect of other variables. complex relationships between variables affecting soil moisture can be understood and predicted by modeling. climate variables such as precipitation or evapotranspiration have a great impact on increasing and decreasing soil moisture. these variables have cause and effect relationship with soil moisture distribution map, so the need for modeling and determination of their role on soil moisture is revealed. the purpose of this study is to evaluate and model soil moisture behavior using two methods of public regression ( OLS ) and geographically weighted regression ( GWR ) and evaluate the accuracy of the models using model validation indexes. Materials and Methods Fars province is located in the south of the central region of Iran between the latitudes of "42 '31 °,27 to" 23 '37 °,31 north to "14 '32 °,55 to" 41 '30 °,50 east with an area of 122. 799 square kilometers. In this study, ECMWF database data were used for spatial analysis and modeling. The data of this database, starting from 1979, is becoming more complete every month, so that at the time of writing until April 2018, it has been released and ready to be downloaded. In this study, according to the studies of previous researchers and geographical studies, as well as cognition obtained from the spatial territory of the study area, the middle of rainfall climatic variables as the most important climatic variable-affecting soil and water moisture equivalent to snowmelt because part of the study area is mountainous. It was extracted after melting mountain snow and the resulting runoff could be a good source of soil moisture. The mean actual evapotranspiration layer was also obtained from ECMWF database data. A moisture topography variable that is used for quantitative studies of watersheds is a good indicator of soil moisture status. Regression modeling allows the relationship between independent and dependent variables to be identified and quantified. In ordinary linear and nonlinear regressions, it is assumed that the independent variables are the same throughout the study area, in reality this is not the case. In spatial regression, the coefficients of the independent variables are calculated to different degrees and it is assumed that they have more weight in places close to the complication. Among the regression models, the ordinary least squares regression method is the most common and simplest method. In spatial modeling by the OLS method, it is assumed that the coefficients or parameters of the statistical model are fixed to a place (geographical coordinates),therefore, the value of the dependent variable that is estimated by this model is the same for all parts of the region, which is considered as a weakness of this method in spatial modeling. There are several indicators to evaluate the validity and efficiency of regression models, some of which were used in this study: Coefficient of determination (R 2) and AICC criterion method (AICc). Results The average soil moisture in the study period was extracted as a dependent variable in modeling. The highest value is related to the northwest and the lowest value is related to the south and northeast of the province. The rainfall distribution was extracted as the most important independent climatic variables in soil moisture distribution in the study area shows the highest rainfall from mid-autumn to early spring and can indicate the maximum frequency of rainfall systems in this time range. Due to the mountainous nature of some areas, water equivalent to snow was studied as one of the independent variables. The median of evapotranspiration was also estimated as an independent variable. Vegetation indices may also indicate soil moisture. In this study, the average NDVI index in the study period extracted from Landsat 8 satellite images was obtained using the Google Earth Engine system. the province's rainfall, the vegetation index in the west and northwest is the maximum, and as it goes east and south of the province, the amount of this index decreases. By examining the equivalence of the independent variables by taking the correlation between the five independent variables and also examining the increase in the inflation index of VIF variance and examining the rate of change of regression coefficients by deleting or adding individual variables in a multivariate regression model, by performing about 6 transformations of events, the best transformation of the natural logarithm (ln (x)) was identified and used. Then the general least squares error (OLS) regression fitting was performed on the data. P-estimated = 1/75 + 0. 024416WI + 0. 3208 PR-0. 0164AET + 0. 002811NDVI + 0. 000724SWE Equation seven is a linear relationship between independent and dependent variables according to which independent variables can to justify and explain 66. 59% of soil moisture changes. According to the general regression. The rain variable layer has the greatest effect among the 5 variables used in modeling on soil moisture. In other words, with each millimeter of increase in rainfall, the soil moisture in the province may increase by an average of 0. 32 cubic meters per cubic meter. The second effective variable of topographic moisture index was extracted, which ranged from one to 0. 03, and by comparing this layer with the slope, it can be seen that wetter areas of the soil in the west and northwest of the province are fed by upstream moisture, which have more slope, while areas with moisture. Less east and northeast of the province, because of rainfall, are directed downwards. Evapotranspiration has a decreasing effect on soil moisture, because one of the ways to get water out of the soil is by using plants called transpiration and water consumption by evaporation from the surface of the soil. The next variable of water is equivalent to snow. The effect of this variable has shown itself more due to the snow-covered western and northern regions of the province. The vegetation variable has the lowest coefficient of rainfall in the general regression equation due to slow growth and delayed reaction. One of the most important factors for water penetration in the soil and creating moisture in it is the soil texture, i. e. its constituents of clay, silt and sand. In this research, a lot of effort was made to create a soil texture layer, but due to the size of the study area and the need for many samples for testing, this layer was not considered. Regarding the vegetation layer in this study, the average interval studied was considered. Conclusions In the present study, to identify the relationships between spatial factors and soil moisture dispersion, using the generated information, the average of each factor in each cell and according to the spatial characteristics of each cell, using conventional regression (OLS) and geographic balanced regression (GWR) techniques. Was modeled. First, a logarithmic transformation was performed on the data to remove the alignment and reduce the inflation index of variance, and then the general regression equation was fitted to the data. In this study, the results of geographic rhythmic regression model showed that the highest soil moisture is seen in the northwest. The most negative effects of evapotranspiration are seen in the east and southeast. Based on the error scattering map, the spatial regression model was able to explain more than 50% of soil moisture changes in more than 68. 04% of the area. Model residual maps also show a decrease in the amplitude of the GWR model residuals compared to the OLS model residues. However, the error rate was much lowerThe main advantage of spatial weighted regression method over the conventional regression method is its ability to study the spatial effect of variables. Finally, it can be said that using GWR forecasting maps, areas prone to significant decrease or a significant increase in soil moisture in Fars province can be identified and used to improve the decisionmaking process and forecast the service needs of relevant agencies. To improve this modeling of soil moisture, it is suggested to use the texture map and soil type as an independent layer in the modeling. Depth 10-12 cm (penetrates) to be used as independent variables. It is also recommended to use radar images that have high resolution to extract the moisture layer with high resolution for more detailed examination.