Wet trend doesn't forestall droughts, expert says

Northwest China has become warmer and wetter, but the change remains too limited to fundamentally alter the region's arid climate or resolve its water shortages, a leading Chinese hydrologist said on Saturday.

Zhang Jianyun, an academician of the Chinese Academy of Engineering and former president of the Nanjing Hydraulic Research Institute, made the remarks at the IWHR Water Science Conference held at the China Institute of Water Resources and Hydropower Research in Beijing.

Zhang said warming in Northwest China is unequivocal. Temperatures have risen amid fluctuations over the past century, with the increase exceeding the global average.

The region has also experienced a general increase in precipitation, but the timing and extent vary geographically. Western parts of Northwest China have seen precipitation increase for about 35 years, while the wetter trend in eastern areas emerged only after 2000.

However, Zhang cautioned against exaggerating the trend or interpreting it as evidence that Northwest China is becoming a warm and humid region. Precipitation in the western part has increased by only about 10 to 20 millimeters per decade, while the region's overall precipitation remains roughly comparable to levels recorded in the 1960s.

"The increase represents a quantitative change and is not sufficient to transform the region's fundamentally cool and arid climate," he said, adding that Northwest China is also unlikely to develop a warm and humid climate in the foreseeable future.

Higher temperatures have accelerated the melting of mountain glaciers and snow, contributing to increased water flows and changing the volume and seasonal distribution of runoff in rivers mainly replenished by meltwater, he said. However, warming has also intensified evaporation, offsetting a considerable share of the benefits brought by increased precipitation.

Recent droughts demonstrate that a wetter long-term trend does not eliminate the risk of severe water shortages, he stressed.

Zhang said that persistent heat and low rainfall beginning in June, for example, caused drought conditions in parts of Gansu province. In August, the situation triggered the upgrading of drought emergency responses from Level IV to Level III in five cities, which included Jiuquan, Zhangye and Wuwei, while Jiayuguan directly issued a Level III emergency response. Some northern parts of the Hexi Corridor recorded 50 to 90 percent less precipitation than normal.

China has a four-tier emergency response system, with Level I being the most severe response.

Water storage at Hongyashan Reservoir in Minqin county, which is overseen by Wuwei, fell to about 14 percent of its normal level, approaching its dead-storage level, Zhang continued.

He also noted that natural runoff in the Yellow River Basin has continued to decline under the combined effects of climate change and socioeconomic development. Successive assessments put its average annual natural runoff at 58 billion cubic meters for 1919-1975, 53.5 billion cu m for 1956-2000 and 49 billion cu m for 1956-2016.

Zhang called for a scientific and comprehensive understanding of the warming and wetting trend, and warned that it should not be used alone as a basis for major long-term water resource and infrastructure decisions.