HYDROLOGICAL REGIME AND HYDRAULIC CHARACTERISTICS OF THE DARYALYK MAIN COLLECTOR SYSTEM (KHOREZM REGION, UZBEKISTAN): LONG-TERM RECORDS AND FIELD FLOW MEASUREMENTS

The Daryalyk main collector system conveys saline drainage water and return flow from five districts of the Khorezm region (Uzbekistan) across the state border to Turkmenistan. Quantitative information on its flow regime and hydraulic behaviour is needed for planning cleaning, reconstruction and possible reuse of drainage water. This study combined (i) a 21-year record (2004–2024) of mean annual discharge, runoff and mineralization; (ii) frequency analysis of mean annual discharge with the Pearson Type III distribution; (iii) acoustic Doppler (SonTek S5, GPS-aided) discharge measurements at five cross-sections; (iv) hydraulic-resistance experiments on three 150 m reaches of the Divankul collector; and (v) clamp-on ultrasonic flow measurements in the pressure pipes of five pumping stations, complemented by 2021–2024 records of pumped volume and electricity use. Mean annual discharge was 22.98 m3/s (Cv = 0.35, Cs = 0.27; range 8.91–41.54 m3/s) and mean mineralization was 2.12 g/L (1.66–3.16 g/L). Discharges exceeded in 50, 75 and 96% of years were 22.6, 17.4 and 9.7 m3/s. Mean discharge in 2018–2024 (17.6 m3/s) was about 31% lower than in 2004–2017 (25.7 m3/s), although the trend was not statistically significant. Measured discharge decreased from 52.8 m3/s at the border cross-section to 1.6 m3/s in the uppermost one. In three Divankul reaches, Reynolds numbers of 3.4–3.8 × 105 indicated fully turbulent flow, and water-surface slopes of 1.1–1.5 × 10−3 at velocities of 0.32–0.36 m/s implied very high flow resistance (Manning's n ≈ 0.11–0.13). The pumps delivered 85–95% of their design discharge of 0.5 m3/s. Pumped volume in Gurlan district fell from 3.62 to 0.15 million m3 between 2021 and 2024 at an almost constant 0.078 kWh/m3. The results give a quantitative basis for prioritising cleaning and for assessing the performance of water-lifting installations on the collector system.