Available water volume from the regional sources is ever declining sharply under extreme drought. While the drought-relief water-supply tasks can be assigned to rapidly increase the major reservoirs. However, the conventional reservoir operation cannot fully meet these harsh demands in recent years. It is often required for the high effectiveness of drought-response, in order to buffer the severe water-supply shortages. In this study, a demand-oriented perspective was adopted to distinguish between normal and extreme drought scenarios. A graded drought-limited water level (DLWL) was dynamically controlled to consider the historical supply-demand patterns and abrupt surges in the water demand during extreme events. Three primary steps were set: 1) To define the supply-demand boundaries for the different drought scenarios, according to the water-use patterns and long-term hydrological statistics, 2) To calculate the regular and special DLWLs that represented the minimum storage thresholds for the normal and extreme drought demand, respectively, and 3) to formulate the dynamic control for the switching between the regular and special DLWLs, according to the drought severity, reservoir storage evolution, and the projected surges of the water demand. A case study was conducted for the Wangkuai and Xidayang reservoirs, as well as the Shahe and Tanghe irrigation districts in Hebei Province, China. The regular-special DLWL was calculated to develop a dynamically switchable strategy of the graded DLWL control, in order to guide the drought-response operations. The regular DLWL mode was applied to the normal drought conditions, in order to fully meet the water supply for the urban domestic use, agricultural irrigation, and ecological baseflow. The special DLWL mode was activated under extreme drought. The urban domestic water supply was prioritized to moderately restrict the agricultural irrigation, and then further limit the ecological water use. Three scheduling schemes (baseline scheme, rugular scheme and proposed scheme) were implemented for the comparative analysis. The evaluation was focused mainly on the extremely dry years and typical multi-year dry periods. The results indicate that the regular-special DLWL scheme was applied to substantially mitigate the water shortages in the extremely dry years. Specifically, the total water shortages decreased by 66.53 million and 32.45 million m³, respectively, in the Shahe and Tanghe irrigation districts. The earlier strategic storage triggered by the special DLWL markedly improved the water supply guarantee rate during several severely impacted periods in the typical consecutive dry years. There was also an increase from below 10% to approximately 90% and 50%, respectively. The irrigation shortages declined by 186.20 million and 182.74 million m³, respectively, over the multi-year droughts. Furthermore, the number of months with the severe shortage decreased by 7 and 32 months, respectively, effectively alleviating the deficits. There was a decrease in the frequency and duration of acute water-stress episodes. Moreover, the approach was more effective in transforming the extreme shortage into a wider and shallower shortage pattern. The incidence of the high-intensity shortage episodes was also reduced to lower the risk of severe shortages. The guarantee rate for agricultural irrigation was improved after optimization. The findings can also provide scientific support for the reservoir drought resistance and emergency decision-making under extreme drought.

Aiming at the problem of determining the drought-limited water level of emergency drought-resistant backup reservoirs, a model for determining the drought-limited water level was established based on the principle of wide and shallow damage with the goal of minimizing the time period average water shortage rate and the number of time periods with severe water shortage. A method and operational rules for determining the drought-limited water level of emergency drought-resistant backup reservoirs under extreme drought conditions were proposed. The proposed method was applied to the Deze Reservoir, a backup water source for Kunming City, and the degree of water supply guarantee under different schemes. The results indicate that the method can realize the optimal design of phased drought-limited water level in emergency drought-resistant backup reservoirs, and effectively reduce the adverse impact of extreme drought on socio-economic water use. By setting the drought-limited water level of the Deze Reservoir, the number of months with severe water shortage for emergency water supply in Kunming City can be reduced from 229 to 34, and the average monthly water shortage rate can be reduced from 27.58% to 12.74%, thereby mitigating the occurrence of extreme water shortages. Under extreme drought conditions, the combined use of the drought-limited water level and dead storage capacity can further significantly enhance the level of water supply guarantee, reducing the number of months with severe water shortages to 21 and the average monthly water shortage rate to 2.52%.

In view of the problems such as strong subjectivity and insufficient rationality in determination of water demand according to the empirical adjustment coefficient in calculation of the drought-limited water level in dry periods, this paper proposes a reservoir drought-limited water level optimization method based on the principle of wide and shallow damage, to optimize water demand of different users with the aim of minimum mean monthly water shortage ratio and its standard deviation in dry years. Taking the Fenhe Reservoir and the Fenhe-Ⅱ Reservoir as examples, optimization effect analysis was carried out to verify the rationality and feasibility of the method. The results show that the mean monthly water shortage ratio and its standard deviation in dry years can be reduced by using the optimized drought-limited water level to guide reservoir operation. Taking 50% as the threshold for severe water shortage ratio, the optimized drought-limited water level can effectively reduce the number of months with severe water shortage. In consecutive dry years, the reservoir scheduling effect under the control of the optimized drought-limited water level is significantly improved, compared with that under the control of the non-optimized drought-limited water level, and the optimized drought-limited water level can reduce the frequency of severe water shortage, meeting the requirements for wide and shallow damage in dry years. The setting of the drought-limited water level can ensure water safety of various users in dry years, reduce the occurrence of extreme water shortage, and provide a basis for drought prevention command and decision-making.