1. Introduction
Climate change has significant impacts on hydrological, biological, and ecological systems which are all closely related to water resources, causing sustainability concerns around the world. The Fifth Assessment Report (AR5) of the Intergovernmental Panel on Climate Change (IPCC) confirms that warming in the climate system is unequivocal, with many of the observed changes unprecedented over decades to millennia. Temperatures at the Earth’s surface in each of the last three decades has been successively warmer than that in any preceding decade since 1850 [
1]. These facts indicate that future climate change mainly characterized by warming and tightly related with socio-economic development will be of significant global, national and regional importance. Understanding the evolution mechanism of climate change has thus become a priority area, both for research and for water management strategies. In the past 100 years, the increase of the land surface air temperature in China was greater than that of the whole world, especially in the inland regions of Northwest China with an increase >0.5 °C since the 1980s [
2]. Compared with temperature, precipitation and its influencing factors are more complex and uncertain with no consistent trends in Northwest China, which cause much more vulnerability and sensitivity of agricultural water consumption and productivity to climate change [
3].
In terms of the Representative Concentration Pathways emission scenarios (RCPs) according to the radiative forcing target level for 2100, general circulation models (GCMs) provide multiple global large-scale climate information for impact assessment, however, with a relatively coarse spatial resolution. Facing this problem, downscaling methods have been developed to generate higher-resolution climatic factors for regional studies based on GCM outputs [
4,
5]. The Statistical Downscaling Model (SDSM) developed by Wilby et al. [
6,
7,
8] has been widely used to investigate the impacts of regional climate change on water resources. Fan et al. [
9] demonstrated that the SDSM showed a high efficiency for simulating the air temperature in Northern China, indicating a significant trend of increasing air temperature in the future. Compared with the high accuracy of the air temperature, the simulated monthly precipitation by the SDSM exceeded the observed value [
10]. Hao et al. [
11] applied the SDSM to project the spatio-temporal characteristics of the future air temperatures and precipitation in the Hexi Corridor, implying that the daily maximum, minimum, and mean air temperature (T
max, T
min, T
mean) all exhibited an increasing trend, while the precipitation showed significant regional variations, declining in the eastern and central parts and increasing in the western part. However, these previous studies have neglected the performance assessment on GCM selection in terms of specific study areas, which could cause great uncertainties in the regional impact evaluation of climate change [
12,
13]. To solve this problem, Wang et al. [
14] firstly evaluated the 23 GCMs’ performances in the HRB, and then constructed the SDSM to provide the variation ranges of different climate variables simulated by multi-GCM under multiple scenarios, providing a reference for comparison between different GCMs and the concurrent issues related to downscaling. Climate change scenarios generated by Wang et al. [
14] were used in this study.
As the second largest inland river basin in the arid region of Northwestern China, the HRB plays an important role for the production of commodity grains and the development of the Belt and Road initiative. Both the agricultural irrigation in the middle oasis and the fragile ecosystems in the downstream basin strongly depend on Heihe River water. However, the irrigation water diversion of the middle oasis accounts for more than 80% of the river runoff at Yingluoxia [
15]. To restore the ecosystems in the downstream HRB, the Ecological Water Diversion Project (EWDP) has been applied since 2002. According to the EWDP, the river water allocated to the middle oasis is to be significantly reduced for increasing runoff discharge. Yet, the total water use is actually not reduced in the middle oasis [
16] due to inappropriate land and water use and management [
17,
18]. More groundwater is exploited to supplement irrigation. This has resulted in a declining trend in groundwater levels [
19,
20] and the shrinkage of wetland and grassland areas [
21] in the middle oasis. Therefore, how to optimize the water allocation ratio to the middle oasis and increase the agricultural water productivity becomes a challenge to the HRB [
22].
Studies concerning hydrology, ecology, and economy have been accordingly conducted to reveal and synergize the complicated relationship of water-agriculture-ecology in the HRB. Kang et al. [
23] demonstrated that water availability and crop production were likely to decrease in the future in response to increasing temperatures and fluctuating precipitation. Cao et al. [
24] conducted a study on corn in the oasis irrigation areas of the Hexi Corridor indicating that the accumulated temperature in irrigated areas in the Hexi Corridor was significantly increased. The accumulated temperature during corn growth is a key factor affecting corn yield. As the climate in the irrigated areas warms, the corn yield in selected local areas has increased. The yield increases, from west to east, were 124%, 186%, and 301%, respectively, however, only one climate factor (temperature) was taken into consideration in this study. Zhao et al. [
25] found that the air temperature in the HRB increased significantly from 1960 to 2009, which was closely correlated with the increasing yields of spring wheat, corn, and cotton, but higher pest population levels. Zhang et al. [
26] investigated the responses of crop water use efficiency (WUE) to climate variables in the semi-arid area of Northern China during 1983–2010, suggesting that the difference of crop WUE in warm-dry environment and in warm-wet environment ranged from 29.0–55.5%. Changes in temperature and precipitation in the past three decades jointly enhanced crop WUE by 8.1–30.6%. Jiang et al. [
17] developed a distributed agro-hydrological model to quantify the combined effects of weather, crop, soil and irrigation factors on irrigation performance and water productivity in the irrigated areas of the milled HRB, which implied that the water productivity was spatially varied and quite small due to excessive irrigation water use. Xu et al. [
27] applied a distributed agro-hydrological model to access the irrigation water use in the major irrigation system of middle HRB, indicating that only 53% of total applied water was efficiently used, whereas deep percolation loss and canal conveyance loss accounted for 22% and 25% of the total applied water, respectively. It has been shown that since the late 1980s, the frequency and intensity of extreme hydrological-meteorological events have increased in Northwest China, which has caused serious adverse effects on local livelihood, such as agricultural production, water resources sustainable management, and food security, and has restricted the regional socio-economic development [
28,
29,
30,
31]. To sum up, this study aims to evaluate the impacts of climate change on water productivity in the irrigated oases of the middle HRB by: (1) using multi-GCM projections which could effectively reduce uncertainties from GCMs; (2) applying an agro-hydrological model to simulate the crop growth based on different climate change scenarios and crop reference evapotranspiration (
ET0) scenarios derived from the Hargreaves equation adjusted by the Penman–Monteith equation; and (3) selecting two typical agricultural production areas to quantify variation patterns of the crop water productivity. Results obtained in this study will be helpful for synergic development of water security and agricultural production in arid inland regions adapt to climate change.
4. Conclusions
In this study, the multi-GCM projected climate change under the scenario of RCP2.6, RCP4.5, and RCP8.5 coupled with an agro-hydrological model SWP-EPIC was proposed to evaluate the water productivity of maize in the middle irrigated regions of the Heihe River basin from 2021–2050.
Compared with the reference period, the result show that precipitation was predicted a decrease under the future climate scenarios, while the Tmean, Tmax, and Tmin all exhibited increasing trends. The increase of the Tmax was greater than that of the Tmean and Tmin. Driven by the multi-GCM projections of climate change, the maize growth in the Gaotai and Ganzhou areas was simulated to obtain the yield, CWC, and WP. Compared with simulation results based on field experiments from 2012–2015, the WP in the two typical agricultural areas both exhibited an increasing trend, which was mainly attributed to decrease of crop water consumption. The water productivity under three RCP scenarios in the Gaotai area during 2021–2050 increased by 9.2%, 14.3%, and 11.8%, and which increased by 15.4%, 21.6%, 19.9% in the Ganzhou area, respectively, most of which were larger than 2 kg/m3, indicating that water-heat-carbon projected under the RCP4.5 scenario in the middle irrigated areas of the HRB synergized the best for maize growth. Although the irrigation schedule in this study was unchanged, implications of the relevant water saving practices for the irrigated areas of the middle HRB could also be obtained from the perspective of the increase of efficiency in the process of field water application. The specific suggestions emphatically include: (1) improving traditional irrigation methods and concepts of field water use (using the modernized basin, prompting the water-saving education for farmers); (2) implementing water-saving irrigation technologies and equipment in areas with higher income or powerful financial support. The findings can provide useful information on the Hexi Corridor and the Belt and Road to policy-makers and stakeholders for sustainable development of the water-ecosystem-economy system.