*3.1. WESVs and Characteristics in 2010–2020*

3.1.1. Temporal and Spatial Variation Analysis of WESV

Taking the county-level administrative region as the minimum calculation unit, and adopting appropriate methods according to different service types, the WESVs of typical areas in the middle of the Loess Plateau in 2010, 2015 and 2020 were calculated (Table 3). The changes in counties among the three years have also been shown (Figure 2). Overall, the WESV in the study area showed a trend of first decreasing and then increasing. The WESV decreased from 12,370.42 million CNY in 2010 to 11,746.86 million CNY in 2015, which showed a drop range of 5.04%. Then, it increased to 13,379.48 million CNY in 2020. The increasing range was 13.90% from 2015 to 2020, and 8.16% during the entire study period of 2010–2020.

It can be seen from Table 4 that, from 2010 to 2020, some districts and counties in Yan'an City in the southern part of the study area experienced a decrease in WESV, but the trend was rising, with a rate of 3.96%. The leading area that led to the decreasing trend in 2015 was Yulin City. From 2010 to 2015, the WESV in Yulin City decreased by 7.84%, which was significantly greater than the entire study area decreasing rate. Only four counties of Yuyang, Hengshan, Mizhi and Zizhou showed an increase in WESV, and the remaining two-thirds of the counties presented a decreasing trend; the decreasing degree in Fugu reached the highest 30.46%. These phenomena indicated that the ecological service function provided by water resources in Yulin City was in the stage of degradation from 2010 to 2015, which also indirectly reflected the trend of ecological environment deterioration.

From 2015 to 2020, WESV in Yan'an City was still in a growth trend as a whole, and the growth rate increased to 8.56%. Only Yanchang, Ansai and Ganquan experienced negative growth. During the same period, WESV in Yulin City increased rapidly, with an overall increasing rate of 15.77%, and 75% of the counties in the jurisdiction were on the rise, mainly due to the low value of WESV in 2015, which showed that the water ecosystem and its service functions in Yulin City were in a significant recovery situation in 2015–2020.

From the whole study period, the increasing rate of WESV in the southern part of the study area was significantly greater than that in the northern part. In 2020, WESV in Yan'an increased by 12.86% compared with 2010, while Yulin increased by only 6.69% in this decade.

**Figure 2.** Changes of WESV among the three years in different regions.


**Table 3.** WESV of the typical area in the central Loess Plateau. unit: million CNY.


**Table 4.** Structural of WESV in 2010, 2015, 2020.

3.1.2. Variation Analysis of WESV Structural Characteristics

Table 4 and Figure 3 exhibited the changes in the value and proportion of nine subservices that comprise WESV from 2010 to 2020. Among the three first-level classifications of WESV, Regulating Services accounted for the largest proportion, but the proportion gradually decreased from 86.55% in 2010 to 68.44% in 2020. The Provisioning Services proportion has increased year-by-year, from 10.73% to 29.42% in the past decade.

**Figure 3.** Structural changes of WESV in the typical area of the central Loess Plateau: (**a**) the proportion of various WESV in 2010; (**b**) the proportion of various WESV in 2015 and (**c**) the proportion of various WESV in 2020.

The Supporting Services proportion has decreased steadily, from 2.72% in 2010 to 2.14% in 2020.

In the subcategories, only the proportions of Water Supply and Food Production have increased year-by-year, and the proportions of other ESV have decreased to varying degrees, which showed that the development of economy and society has caused a sharp increase in the demand for water resources and aquatic products, and at the same time, the regulating function of water resources has gradually weakened.
