## 4 Discussion
Therefore, it is necessary to regionalize and inter-annualize the \(D\) value to adapt to the changes in the ecological background and socioeconomic development level in the study area and study year, especially in China's large cities, with the rapid development of the social economy and urbanization. Based on the EFM of [PERSON] et al., this paper proposes the regionalization and interannual modification methods of the \(D\) value. In the method, the structure of the \(D\) value modification coefficient section was consistent with [PERSON]'s equation. In terms of the calculation content, the three main crops in the country in [PERSON]'s equation were replaced by the three main crops in the study area, and the interannual modification of the \(D\) value was realized by calculating the statistics of different study years. The crop data were compiled from statistics and agricultural assembly documents of China and Guangdong Province, and the calculation maintained the consistency of the reference basis and statistical caliber of [PERSON]'s research. This study used the difference in crop planting structure and its economic benefit in the study area to modify the \(D\) value in the study year and study area, and it was an improvement of the EFM; however, the improvement was still insufficient. Although the changes in the economic benefits of the main crops largely reflected the changes in agricultural productivity and socioeconomic development levels, the proportion of economic benefits of crops in the GDP was very low for economically developed regions. For example, Guangdong Province, where the study area is located, is one of the most developed and densely populated regions in China. With rapid socioeconomic development, Guangdong residents have increasingly higher requirements for the ecological environment where they live. Therefore, the demand for ecosystem services in Guangdong is enormous. However, the agricultural economy of Guangdong Province is underdeveloped, the economic benefit of crops per unit area is weak, and the per capita agricultural output value is only 73% of the average national level. In 2016, the total agricultural output value of Guangdong Province accounted for only 4.65% of the GDP, which was lower than the average national level of 8.56%. During 2004-2016, the growth rate of Guangdong's total agricultural output value (197%) lagged far behind GDP growth (396%) in the same period. Under the background of a large contradiction between ecological supply and demand and weak agricultural benefits in Guangdong Province, the \(D\) value modified by the economic benefits of grain crops may be on the low side, which is obviously inconsistent with the economic structure dominated by second and third industries in Guangdong Province. Therefore, it is necessary to introduce the statistics of the GDP, CPI and other indicators of the level of social and economic development into the modification of the \(D\) value in EFM in future research [51]. Urbanization profoundly changes the natural landscape of a region. In essence, it is the process of continuous transformation from regional natural ecosystems and agricultural ecosystems to urban ecosystems, which affects or irreversibly changes the structure, process and function of ecosystems [52]. As the increases in forest, grassland, wetland and other ecological lands in the study area cannot compensate for the loss of ecosystem service function caused by the massive expansion of built-up land, this paper presents the net decrease in the total ESEV in the study area. The ESEV, which is used to characterize the ecosystem service function and capacity in a region, is affected by climate, ecological background quality and various ecological land area changes. The change in the total ESV is based on the change in the total ESEV, which is also affected by the change in the unit ESEV standard value brought about by socioeconomic development.