Calculation of instream ecological water requirements under runoff variation conditions: Taking Xitiaoxi River in Taihu Lake Basin as an example

doi:10.1007/s11442-021-1889-x

Abstract

Abstract:

Climate change and human activity can cause remarkable hydrological variation. Traits of hydrological series such as runoff before and after the change points could be significantly different, so the calculation of instream ecological water requirements (EWRs) is confronted with more challenges. Taking the Xitiaoxi River (XTXR) in the upper reach of the Taihu Lake Basin as an example, this paper investigates the calculation of EWRs using the range of variability approach (RVA) under changing environment. The change point diagnosis of the natural and observed runoff series are conducted for XTXR. Then, differences in the hydrological alternation indicators and instream EWRs processes obtained from various daily runoff series are compared. It was found that the natural and observed annual runoff series in XTXR from 1957 to 2018 both show significant variations, and the change points are in 2007 and 1999 respectively. If runoff data before the change points or all runoff data are used, the instream EWRs obtained from natural runoff are significantly lower than those obtained from the observed runoff. At the monthly time step, EWRs differences within a year mainly occurred from May to August. Also, calculation results of the instream EWRs are strongly related to the selected period of runoff series. The EWRs obtained using runoff series after the change points have rather acute fluctuation within a year. Therefore, when the RVA method is used under changing environment, the instream EWRs should be prudently determined by comparing different calculation results on the basis of river runoff restoration and variability analysis. To a certain extent, this paper enriches our understanding about the hydrological method for EWRs estimation, and proposes new ideas for future research on EWRs.

Key words: runoff variation, Taihu Lake Basin, Xitiaoxi River, ecological water requirements, RVA

WANG Qin, WANG Shuwen, HU Qingfang, WANG Yintang, LIU Yong, LI Lingjie. Calculation of instream ecological water requirements under runoff variation conditions: Taking Xitiaoxi River in Taihu Lake Basin as an example[J].Journal of Geographical Sciences, 2021, 31(8): 1140-1158.

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IHA indicator calculation results at HTCS

IHA indicator	Hydrological parameter	Observed runoff				Natural runoff
		Mean	Cv	RVA threshold value		Mean	Cv	RVA threshold value
		Mean	Cv	Upper limit	Lower limit	Mean	Cv	Upper limit	Lower limit
Monthly mean flow	Mean flow in January	15.16	0.67	20.17	7.71	22.94	0.49	30.17	14.74
	Mean flow in February	21.90	0.70	28.82	9.51	29.76	0.46	39.92	16.82
	Mean flow in March	35.37	0.52	46.92	21.26	39.30	0.46	48.57	26.09
	Mean flow in April	35.20	0.49	42.94	22.42	40.12	0.38	49.92	29.14
	Mean flow in May	38.34	0.65	48.52	17.99	42.98	0.40	51.71	31.49
	Mean flow in June	51.28	0.88	59.48	18.12	61.52	0.57	69.76	37.52
	Mean flow in July	48.18	0.77	64.55	23.43	57.77	0.51	70.95	37.89
	Mean flow in August	42.80	0.85	64.18	14.17	57.39	0.58	78.22	32.53
	Mean flow in September	43.13	0.79	55.65	20.10	55.94	0.56	70.60	37.22
	Mean flow in October	27.22	1.08	27.78	9.12	35.87	0.69	46.14	19.81
	Mean flow in November	16.66	0.78	20.67	9.31	27.40	0.57	35.38	17.49
	Mean flow in December	14.71	0.72	20.27	6.83	21.48	0.55	25.67	14.47
Annual mean extreme values	Min. 1-day flow	1.82	1.61	1.84	0.00	6.89	0.30	8.65	5.27
	Min. 3-day flow	2.34	1.32	2.76	0.07	7.04	0.31	8.81	5.35
	Min. 7-day flow	2.98	1.08	4.05	0.19	7.44	0.31	8.98	5.74
	Min. 30-day flow	5.80	0.69	7.17	2.98	11.39	0.35	14.45	8.41
	Min. 90-day flow	13.15	0.53	16.21	8.43	22.13	0.34	27.39	16.45
	Max. 1-day flow	492.06	0.51	679.00	308.00	463.33	0.54	581.41	277.67
	Max. 3-day flow	352.80	0.51	477.33	205.00	354.33	0.51	438.15	220.54
	Max. 7-day flow	238.82	0.52	302.86	138.86	224.20	0.44	289.10	145.50
	Max. 30-day flow	108.69	0.46	137.18	75.04	110.04	0.35	141.66	82.53
	Max. 90-day flow	64.31	0.38	80.67	49.23	71.55	0.31	88.45	54.07
	Number of days with zero runoff	7.60	2.44	3.00	0.00	0.00	0.00	0.00	0.00
	Base flow indicator	0.09	0.97	0.14	0.01	0.18	0.24	0.22	0.16
Annual extreme value occurring time	Max. flow occurring time	198.48	0.34	244.00	166.00	202.61	0.28	245.00	174.00
Annual extreme value occurring time	Min. flow occurring time	208.48	0.57	312.00	92.00	190.90	0.78	346.00	39.00
High and low flows and duration	Low flow trough value	14.35	0.30	17.00	11.00	18.13	0.24	21.00	15.00
	Low flow mean duration	13.32	0.83	18.00	5.00	9.23	0.40	11.00	6.00
	High flow peak value	6.33	0.36	7.45	4.93	5.45	0.25	6.05	4.56
	High flow mean duration	7.60	0.64	10.14	4.00	9.00	0.35	11.00	6.73
Flow changing rate and frequency	Water rising rate	118.56	0.25	129.00	98.00	78.27	0.09	84.00	73.00
	Water falling rate	14.93	0.41	19.27	11.03	17.87	0.29	21.30	14.27
	Number of reverses	10.23	0.37	12.41	7.33	9.56	0.30	11.67	7.25

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Calibration period (1957‒1960)		Verification period (1961-1964)		Parameter calibration value
NSE₁	RE₁ (%)	NSE₂	RE₂ (%)	x₁ (mm)	x₂ (mm)	x₃ (mm)	x₄ (d)
0.69	0.79	0.73	-8.96	1059.8	0.5	30.2	3.3

Division of time periods	No.	Observed runoff	No.	Natural runoff
Time period before variation	O1	1957‒1998	N1	1957‒2006
Time period after variation	O2	1999‒2018	N2a	2007‒2018
Time period after variation	O2	1999‒2018	N2b	1999‒2018
Variation not taken into account	O3	1957‒2018	N3	1957‒2018

Time period	1957-2006 (N1)		2007-2018 (N2a)		1999-2018 (N2b)		1957-2018 (N3)
Time period	Monthly mean flow	Ecological flow	Monthly mean flow	Ecological flow	Monthly mean flow	Ecological flow	Monthly mean flow	Ecological flow
January	21.3	6.0	29.5	8.4	28.3	8.3	22.9	7.7
February	28.0	10.6	37.0	8.5	34.4	11.0	29.8	11.5
March	36.7	10.9	50.1	14.9	44.0	15.1	39.3	11.2
April	38.7	10.5	46.1	13.1	40.9	16.2	40.1	10.4
May	43.9	10.2	39.3	9.0	40.6	10.0	43.0	10.1
June	57.5	14.2	78.5	42.2	70.3	20.0	61.5	16.1
July	54.6	16.3	71.0	11.8	63.8	20.5	57.8	16.5
August	52.4	17.3	78.3	38.8	69.3	35.0	57.4	22.8
September	56.0	18.1	55.9	10.0	48.5	11.0	55.9	16.7
October	31.8	10.0	53.0	31.3	40.3	18.8	35.9	13.2
November	24.4	7.1	39.7	16.7	34.1	13.3	27.4	8.9
December	18.9	3.7	32.1	9.6	28.7	8.8	21.5	5.6
Mean	38.7	11.2	50.9	17.9	45.3	15.7	41.0	12.6

Calculation time period	1957-2006 (N1)	2007-2018 (N2a)	1999-2018 (N2b)	1957-2018 (N3)
Mean value of EWRs	11.2	17.9	15.7	12.6
EWRs Cv	0.40	0.69	0.47	0.38
Max. value of EWRs	18.1	42.2	35.0	22.8
Occurring month	September	June	August	August
Min. value of EWRs	3.7	8.4	8.3	5.6
Occurring month	December	January	January	December

Calculation time period	1957-1998 (O1)	1999-2018 (O2)	1957-2018 (O3)
Mean value of EWRs	13.7	12.2	13.3
EWRs Cv	0.54	0.68	0.48
Max. value of EWRs	25.3	35.5	25.0
Occurring month	July	August	August
Min. value of EWRs	4.8	5.5	5.7
Occurring month	November	February	November