Simulating and understanding effects of water level fluctuations on thermal regimes in Miyun Reservoir

Figures & data

Figure 1. Map of Miyun Reservoir. The circles show the locations of the eight monitoring sites: M1 stands for Baihe station, M2 for Kuxi, M3 for ShuiJiu, M4 for Chaohe, M5 for Taoli, M6 for Henghe, M7 for Kudong and M8 for Jingou station.

Figure 2. (a) Water surface elevation from 1992 to 2011 (MSL stands for mean sea level), and (b) bathymetry curves for Miyun Reservoir (cumulative volume and horizontal area vs water level).

Figure 3. Average and standard deviation (StdDev) series of measured water temperatures at (a) the surface depth (0.5 m), (b) the middle depths and (c) the bottom depths calculated from available measurements at eight different sites M1–M8 (Fig. 1) in Miyun Reservoir for the periods 1998–2005 and 2006–2011.

Figure 4. Inter-annual variation of (a) air temperature at the Miyun and Beijing Weather Stations, (c) total river inflow from 1990 to 2011, and (e) measured and back-calculated outflow for water supply from 1998 to 2011. Frequency distributions (histograms) of these three parameters are shown in (b), (d) and (f), respectively.

Figure 5. (a) Time series of measured and simulated water levels under the scenarios S1, S2 and S3; (b) annual water inputs (direct precipitation, ungauged runoff and river inflow) and losses (outflow and evaporation) in Miyun Reservoir from 1998 to 2011.

Figure 6. Simulated water temperatures at the surface depth (0.5 m) under (a) scenario S1, (b) scenario S2 and (c) scenario S3 compared against mean measured temperatures (including average values ±1 SD) in Miyun Reservoir for 1998–2011.

Figure 7. Simulated water temperatures at the middle depths under (a) scenario S1, (b) scenario S2 and (c) scenario S3 compared against mean measured temperatures (including average values ±1 SD) in Miyun Reservoir for 1998–2011.

Figure 8. Simulated water temperatures at the bottom depths under (a) scenario S1, (b) scenario S2 and (c) scenario S3 compared against mean measured temperatures (including average values ±1 SD) in Miyun Reservoir for 1998–2011.

Table 1. Description of different simulation scenarios.

Simulation scenarios	Description	With inflow and outflow	Reservoir hydrologic model
S1	Baseline (outflows calculated to keep water level unchanged)	Yes	No
S2	Include inflow and outflow process	Yes	No
S3	Include reservoir hydrologic model	Yes	Yes

Table 2. Error parameters computed between measured and simulated temperatures under scenarios S1, S2 and S3.

Simulation scenarios	RMSE (°C)^1			±1×SD (%)^2			±2×SD (%)^2			±3×SD (%)^2
	S1	S2	S3	S1	S2	S3	S1	S2	S3	S1	S2	S3
Surface	1.80	1.83	1.80	40	40	41	59	62	61	78	72	70
Middle	6.37	2.81	2.93	15	45	43	35	70	70	48	78	77
Bottom	4.60	4.02	3.95	19	30	32	41	55	51	55	70	73

¹RMSE was calculated between simulated temperatures and mean measured temperatures derived from available data at the monitoring sites.

²Percentages of simulated temperatures that are within plus and minus 1, 2 and 3 standard deviations (SD) from the mean measured temperatures.

Table 3. Parameters used to define long-term water temperature characteristics in Miyun Reservoir.

Parameter	Description
Maximum surface temperature (Max. Surf.)	Annual maximum daily water temperature at 0.5 m below the water surface
Mean surface temperature (Ave. Surf.)	Annual mean daily water temperature at 0.5 m below the water surface
Maximum bottom temperature (Max. Bot.)	Annual maximum daily water temperature at 0.5 m above the bottom
Mean bottom temperature (Ave. Bot.)	Annual mean daily water temperature at 0.5 m above the bottom
Maximum temperature difference (Max. Dif.)	Annual maximum daily water temperature difference between surface and bottom
Mean temperature difference (Ave. Dif.)	Annual mean daily water temperature difference between surface and bottom
Total stratification days (Str. Days)	Total number of days during the ice-free period when difference between surface and bottom water temperature is larger than 1°C
Maximum temperature day of year (Max. DOY)	Date (day of the year) of the maximum water temperature at the middle or bottom depths
Temperature increase slope (Sti)	Line regression slope of water temperature with respect to DOY during the period from April 1 to the date of maximum water temperature at the middle or bottom depths

Table 4. Simulated annual values of seven parameters (Table 3) used to define long-term water temperature characteristics from 1998 to 2011 under scenario S3.

Year	1998	1999	2000	2001	2002	2003	2004	2005	2006	2007	2008	2009	2010	2011
Max. Surf.	29.34	31.64	31.14	28.89	30.35	28.90	29.20	29.96	29.05	29.15	30.31	29.75	31.71	30.01
Ave. Surf.	15.00	14.54	14.35	14.42	14.21	14.36	14.78	14.69	14.73	15.39	14.73	15.04	14.31	15.22
Max. Bot.	13.21	12.07	15.22	16.37	18.10	19.62	19.65	19.33	19.56	20.39	19.69	19.92	19.37	19.67
Ave. Bot.	7.68	6.62	7.95	8.88	9.23	10.08	10.43	10.31	10.67	10.46	10.34	10.05	10.00	10.46
Max. Dif.	19.61	24.17	21.85	18.67	17.04	15.4	15	17.34	14.84	15.93	14.97	15.71	17.62	16.03
Ave. Dif.	7.32	7.93	6.41	5.54	4.98	4.28	4.35	4.38	4.06	4.93	4.39	4.99	4.31	4.76
Str. Days	240	221	208	206	207	190	206	199	213	205	204	204	197	191

Figure 9. Simulated annual maximum and mean (a) surface water temperatures, (b) bottom water temperatures and (c) differences of temperatures between the surface and bottom depths; and (d) simulated stratification days (Str. Days) under scenarios S1 and S3 in Miyun Reservoir (1998–2011).

Figure 10. (a) Day of the year (DOY) having the maximum water temperatures in the middle depths and bottom depths, (b) slopes of temperature increases during the period from April 1 to the day of maximum water temperatures at the middle depths and bottom depths simulated under scenarios S1 and S3.