1. Introduction
The Tibetan Plateau, also known as the Qinghai-Tibetan Plateau, is the highest plateau in the world, with an average altitude of over 4000 m [
1,
2,
3]; the permafrost over the Tibetan Plateau comprises approximately 70% of all of the permafrost regions in China [
4]. The harsh natural conditions (low temperature and hypoxia) have won this area the reputation as the third pole of the world [
5,
6,
7]. Qinghai Lake, the largest lake in China, is a special ecological function area in the northeastern Tibetan Plateau. Without Qinghai Lake, the sand in Tsaidam would devour most of China and the whole of northern China would become a desert [
8]. Therefore, Qinghai Lake is important for sustaining the ecological safety of the northeastern Tibetan Plateau [
9], because it is not only a natural barrier to prevent the spread of the desertification of the west to the east, but also as it has a significant influence on the climate in the Yellow River Basin [
10]. Thus, the ecological status of Qinghai Lake basin is extremely momentous. Changes in the water level of Qinghai Lake directly affect the ecological system of Qinghai province and even the northwest region [
11], as well as the security of water resources in the Yellow River [
12] and, in turn, the daily lives of people in the region.
A considerable amount of research has focused on the influence of climate change on lakes elsewhere in China [
13,
14,
15,
16] and abroad [
17,
18,
19]. Some of these studies have focused on climatic factors [
20,
21,
22], while others were concerned with human activities [
23]. Climatic factors such as precipitation, evaporation, and temperature variation would affect changes in the lake water level and its hydrological processes. Lee [
4] has used re-tracked Enivsat radar altimeter measurements to generate water level change time series for Qinghai Lake and Lake Ngoring, in the northeastern Qinghai–Tibetan Plateau, and examined their relationships to precipitation and temperature changes. Additionally, human activities such as agriculture, dam construction, reservoir operation etc., may have an impact on changes in the water level. Hu [
24] has concluded that climate change was the main reason for the changes, while human activities have had little influence on Qinghai Lake. Although there have been a number of studies on Qinghai Lake, the effect of artificial forestation and grass planting on the water level of Qinghai Lake has not been considered in those previous studies.
The water level has severely declined from 1959 to 2004 and has recovered from 2005 to 2010, which has stimulated more interest in studying the hydrological conditions of the lake [
25]. The recent rise of the water level in Qinghai Lake is undoubtedly good news for stakeholders in Qinghai Province, China. However, until now, the reasons for lake level changes in Qinghai Lake have not been fully understood; therefore, it is urgent to achieve a full understanding of variations in the lake level, linked with climatic factors and anthropogenic activities. Specifically, the objectives of this study were (1) to reveal the variation in the lake level of Qinghai Lake during the period 1959–2010; (2) to investigate the variation in the climate, human activities, artificial afforestation and grass plantation during the period 1961–2010; and (3) to analyze the cause of the increased lake level in recent years. Additionally, the results can provide a reference for the control structure for the water of Qinghai Lake.
4. Discussion
Water level and its variation play important roles in lake ecosystems [
29]. As the largest inland lake in China, the variation in the water level of Qinghai Lake has attracted a great deal of attention [
30,
31]. In our study, the lake water level was shown to have declined from 1959 to 2000, which was consistent with Li [
32], who came to the same conclusion that the lake water level had tended to decrease in the years from 1959 to 2000. In addition, we found that there appeared to be a significantly increasing trend during the period 2005–2010.
The mean annual water level of Qinghai Lake declined to its lowest level in 2004. Afterwards, the lake level has increased year by year since 2005 and, so far, it has recovered to its level in the 1980s. The rise of the lake level was an important sign for the improvement of the plateau ecological environment in Qinghai Lake. However, the rise of the lake level also resulted in land submergence around Qinghai Lake, especially for the bird island located on the western shore of the Qinghai Lake, which led to continuous limits on bird-breeding. Therefore, effective measures (such as, among others, the construction of an artificial island) should be taken to alleviate the contradiction between the reduction of the breeding ground for birds and the rising water levels in Qinghai Lake.
Climate factors such as precipitation, runoff inflow into the lake, evaporation from the lake, atmosphere and lake water temperature might result in the variation in the lake level [
33]. In recent years, scientists have studied the sediment geochemistry linked to the hydroclimate variability of Qinghai Lake more and focused less, to some extent, on the effects of climatic factors on lake level variation [
34]. As Qinghai Lake was a closed basin with no surface water outflow, the climate factors that had an effect on lake level variation mainly included precipitation over the lake, surface runoff, groundwater inflow into the lake, evaporation from the lake and temperature.
Table 7 showed the Pearson correlation between the changes in the lake level and the climatic factors. Changes in the lake level (
h) were significantly and positively correlated to precipitation (
P), surface runoff water inflow into the lake (
Rs) and groundwater inflow into the lake (
Rg) and negatively correlated to evaporation from the lake surface (
E), with a correlation coefficient of 0.454, 0.559, 0.302 and −0.394, respectively, and all were significant at a level of less than 0.01 (
Table 7). We drew the conclusion that increasing precipitation and surface runoff or decreasing evaporation had a significant effect on the increase of the water level, which mainly occurred from 2005 to 2010. The results were highly consistent with the results of previous studies [
33,
34].
The temperature increase in the basin resulted in an intensified glacier retreat, increased melt water and rising lake level. The reasons for the change in the lake level were roughly similar to the other lakes in the Qinghai-Tibetan Plateau [
35,
36].
Human activities such as human and livestock water use, agricultural irrigation water consumption, and industrial water consumption, may affect the variation in the lake level. The effect of human activities on lake level variation could be reflected by the effect of lake evaporation and precipitation, surface runoff inflow and groundwater inflow.
The average annual water input and lake evaporation were 37.64 × 108 m3, 39.69 × 108 m3 from 1961 to 2010 in the lake, respectively. Meanwhile, the average annual consumption of human activities was 0.74 × 108 m3 from 1961 to 2010. In conclusion, the total water consumption by human activities only accounted for a very small part of the precipitation, surface runoff inflow and groundwater inflow (1.97%) and of the lake evaporation (1.87%) in Qinghai Lake basin. The results revealed that water consumption by human activities had little effect on the water level in Qinghai Lake. Liu et al. (2009) also reported that water consumption by human activities was not the primary cause of the water level in Qinghai Lake.
The average annual water input and lake evaporation were 35.76 × 108 m3 and 33.40 × 108 m3 from 2000 to 2010 in the lake, respectively. The average annual water consumption of artificial afforestation and grass plantation was 1.81 × 108 m3, accounting for 5.07% of the total precipitation, surface runoff inflow and groundwater inflow and 5.43% of the lake evaporation. The proportion was so small that it could be considered negligible. Thus, it was implied that water consumption by artificial afforestation and grass plantation also had little effect on the water level in Qinghai Lake.
In order to prevent and control soil erosion, restore the natural vegetation and mitigate vegetation degeneration, an artificial afforestation and grass plantation campaign has been conducted in Qinghai Lake basin in recent years. In previous studies, some researchers had maintained the position that the implementation of artificial afforestation in the Qinghai Lake basin might reduce the inflow runoff into Qinghai Lake and bring in a sustained decline of the lake water level, due to its high water consumption, canopy interception as well as its reduced soil moisture infiltration. However, in recent years, the rising water level in Qinghai Lake has attracted public attention, and the concerns that artificial afforestation could lead to the decline of the water level might have been eliminated to some extent. On the contrary, to our knowledge, the result showed that artificial afforestation might be conducive to the rising water level of Qinghai Lake.
5. Conclusions
In this study, the variation in the water level in Qinghai Lake, which has a great ecological importance, over a 52-year period from 1959 to 2010 and the variation in climate and anthropogenic factors over a 50-year period from 1961 to 2010 were investigated to evaluate the impacts of anthropogenic and climatic factors.
The results showed that lake level was 3196.55 m in 1959, and fell to 3192.86 m in 2004, with an average decreasing rate of 8.2 cm·year−1 over 45 years. However, the lake level increased by 1.04 m from 2005 to 2010. During the period 1961–2010, the annual average temperature showed an increasing trend in the Qinghai Lake basin, at a rate of 0.32 °C/decade. The annual precipitation showed obvious fluctuations, with an average precipitation of 381.70 mm·year−1, while annual evaporation experienced a decreasing trend (−30.80 mm/decade). The change in lake level was positively correlated to precipitation, surface runoff water and groundwater inflow into the lake, and negatively correlated to evaporation from the lake surface. The results revealed that water consumption by human activities had little effect on the water level in Qinghai Lake. It was implied that water consumption by artificial afforestation and grass plantation also had little effect on the water level in Qinghai Lake.
In conclusion, the water level depended more on climatic factors than on anthropogenic factors, because water consumption by human activities and artificial afforestation and grass plantation accounted for a very small proportion of the average annual precipitation, surface runoff inflow and groundwater inflow and lake evaporation of Qinghai Lake.