Mass and precipitation change in KK and ME from GRACE and GPCP data
GRACE provides time-variable gravity fields, which have been used in various areas of earth sciences [27-29]. We used GRACE data (Sections S1 and S2) to determine the mass changes in the KK and ME (Fig. 2(a-c) and Figs. S1-S2). The mass changes in the ME and KK exhibit opposite trends: the mass change rates in the countries around the ME are negative, whereas the rates over the KK were positive from 2003 to 2015 (Fig. 2a-c). Fig. S4 shows the precipitation (P) from the Global Precipitation Climatology Project (GPCP) [30] and evapotranspiration (ET) from Global Land Data Assimilation System (GLDAS) [31] in the ME and countries around the KK. While Iran is the most irrigated country with the most depleted water storage over the regions (Table S1, Figs. S1-2), the negative trend in P with the positive ET trend (Fig. S3) suggest that the excess water vapor is transported elsewhere.
In addition, GPCP precipitation data show decreased rain in the ME but increased rainfall in the KK (Fig. 2d-f), and the precipitation trends are consistent with the mass change trends in these two regions with correlation coefficients larger than 0.8. Using in situ data at climate stations, Latif et al.[32] showed that the KK precipitation during winter also increased. The above results implied the enhanced precipitation might be associated with the glacier mass gained over the KK during the first decade of the 21st century.
The contribution of irrigation in ME to the glacier mass gain in KK from climate simulation and moisture tracking approach
To show how ME irrigation affects glaciers in the KK, we use the Community Atmosphere Model (CAM) version 5[33] combined with Community Land Model (CLM) version 4[34] to simulate ME irrigation-induced precipitation changes over the KK. We experimented with CAM5+CLM4 using two scenarios: an irrigation run and a control run, both with climatological sea surface temperature and sea ice concentrations represented in the model. In the irrigation run, two additional anthropogenic fluxes, surface water deliveries and groundwater withdrawal are represented in CLM 4.0 globally (Fig. S4). The impacts of irrigation on the KK's glacier mass can be identified by comparing the results from the two simulations. More details on the model experiments are given in Section S3.
Fig. 3a shows that southern Turkey, Syria, eastern Iraq, Iran, Georgia, Armenia, Azerbaijan, Afghanistan, Pakistan, India, southern Uzbekistan, northern Tajikistan, western Kyrgyzstan, and northwestern Xinjiang are heavily irrigated. In the simulations with irrigation applied, the averaged (from April, May, and June) total evapotranspiration over the ME increases, leading to surface cooling, as shown in Fig. 3b-c. Fig. 3d further shows the irrigation induced changes in precipitation and water vapour transport, in which the arrows represent the amount of transported water vapour, while the shaded colour represents the precipitation anomaly. Moreover, the length and direction of the arrow indicate the size and direction of the transported water vapour, respectively, which is the product of the wind speed and the amount of water vapor at 700 mb, where the most enhanced water vapour exists.
Fig. 3d shows that the irrigation run results in positive anomalous precipitation over eastern Afghanistan, the countries around the Pamir Mountains, northern Kunlun Mountain, and Karakoram Mountains. The additional water vapor generated by irrigation is transported through the atmosphere (climatological westerlies) to the region of the Karakorum Mountains, leading to an increase in precipitation. From a significance test on the irrigation-induced excess precipitation (Section S3 and Fig. S5), the mean magnitude of irrigation-induced precipitation gains was approximately 2.0 cm/month with approximately 30% more over the high-altitude region of the KK (the green boundary in Fig. 3d). The portion (in %) here represents the average ratio between the precipitation anomaly and the precipitation from the control run as shown in Fig. S6. Thus, our model simulations suggest that the irrigated water in the ME is evaporated to increase the atmospheric moisture, carried to the KK to increase the precipitation and glacier mass in this region potentially. The positive precipitation anomaly (Fig. 3d) over the Kunlun Mountains in southern Tarim may also contribute to the recent rise of the lake level in western Tibet[35-39]. Interestingly, the precipitation around Tianshan (Fig. 3d) was also increased by ME irrigation, which contradicts the mass loss measured by GRACE in Fig. 2d and Fig. S2[40-41].
We further use the moisture tracking approach (QIBT) [26] (Section S4) to explore where the precipitation is affected by the irrigation-induced water vapour (Fig. 4 and Fig. S7). Fig. 4 shows the contribution of irrigation in ME and India to local precipitation changes in percentage in April, May, and June (0.1 means 10% contribution to total precipitation). The comparison reveals the comparatively insignificant contribution of irrigation in India on local precipitation over the KK. Although the positive irrigation-induced precipitation anomalies over some regions shown in the models (IRR-CTR) are not sufficiently demonstrated in the moisture tracking approach, it further clarifies the physical mechanism underlying the dynamic circulation changes that carry out more water vapor from the west to the KK region, and the positive precipitation anomalies over the KK region are most likely due to the enhanced westerly instead of direct moisture contribution from the irrigation.