Basic and target eco-environment water requirements of a dry inland river under typical flow frequencies in China

Runoff along the Kaqun reach

Monthly runoff along the Kaqun reach (as shown in Fig. 1) was monitored in 1970–2017 by the Kashi Authority of the Tarim River Basin, China.

The annual average BEEWR

The BEEWR at the Kaqun monitoring station and at other reaches was calculated by the improved Tennant method (Mishra, Amrit & Pandey, 2019). The BEEWR of the Yarkand River was 12.08 × 10⁸ m³. The BEEWR of the Yiganqi reach (for the entire year), the 48th Regiment Ferry (April to September), the Alektamu reach (June and September), and the Heiniyazi reach (September) were 9.31 × 10⁸ m³, 2.71 × 10⁸ m³, 1.19 × 10⁸ m³, and 0.44 × 10⁸ m³, respectively. Because BEEWR is the minimum flow maintaining the health of the river, the Yarkand River is a seasonal river, in the non-flood season, it is still necessary to retain a large amount of water to meet the local people’s production and domestic water use. The remaining water is difficult to meet the minimum flow of the river throughout the year, especially the lower reaches of the river. Therefore, for the Yarkand River, below the 48th Regiment Ferry, the remaining water only needs to meet the BEEWR during the flood season or a certain month. BEEWR did not change with total annual flow frequencies in the previously described reaches.

Environmental protection goals and ecological water requirements

Given the season and vegetation characteristics of seasonal rivers, as well as the requirements for ecological water transmission to the Tarim River, three hierarchical ecological protection goals (Fig. 1) were proposed for the area (derived from the Xinjiang Tarim River Basin Kashi Management Bureau). The first protection goal was based on the “ecological protection red line” policy implemented by the Chinese government (Huang & Yan, 2017). The ecological protection red line for natural vegetation in the Yarkand River Basin had been designated as 74.29 × 10⁴ ha. The second protection target was related to the area covered by ecologically sensitive natural vegetation along the lower reaches of the Yarkand River, which totaled 22.27 × 10⁴ ha. The third protection target focused on key types of vegetation, including Populus euphratica forest areas with ecologically sensitive areas. These forests were mainly distributed along both sides of the 320-km-long river channel from Alektamu to Sanhekou (Fig. 1). P. euphratica forest covered 0.09 million ha.

Typically, the ecological water demand for vegetation is calculated using the Area Quota Method or the Diving Evaporation Method (Ye, Chen & Li, 2010). However, the Area Quota Method needs to determine the specific distribution and water quota of different vegetation types. Determining the spatial distribution of different vegetation types is difficult to perform accurately over large areas. However, it is relatively easy to get the data required for the Diving Evaporation Method, such as the annual average depth of groundwater and evaporation, the latter determined using conventional meteorological evaporating dishes (https://data.cma.cn/). Therefore, the ecological water demand of vegetation in different ecological protection areas was calculated using the Diving Evaporation Method. The ecological water demand of vegetation (Y_ecology) in dry areas was calculated as follows: (1) $$Y_{ecology}=W=E·A$$ where E is the evaporation intensity of diving water (Diving water is the groundwater buried in the first stable aquifer below the surface of the earth. Most of the groundwater that is usually seen is diving water), and A is the area of ecological protection obtained from the local watershed authorities, or by referring to the relevant literatures or reports. The parameter E was calculated as: (2) $$E=\mathrm{a}{\left(1-H/H_{max}\right)}^b{E}_{\mathrm{\varphi }20}$$

In formula 2, E is the evaporation intensity of diving water; $E_{\phi 20}$ is an observed value of evaporation from conventional meteorological dishes (obtained from the China Meteorological Science Data Sharing Service Website, https://data.cma.cn/); H is the annual average depth of groundwater, and H_max is the maximum depth of groundwater, with a and b as empirical coefficients, which were assigned values of 0.62, and 2.8, respectively. H_max and H was monitored by using XH17-S1 Telemetry Water Level Meter in the field. a and b can be obtained by consulting local watershed authorities. Using the diving evaporation method, ecological water requirements under the three protection targets have been calculated to be 11.32 × 10⁸ m³, 3.41 × 10⁸ m³, and 1.00 × 10⁸ m³, respectively.

The annual average TEEWRs

We calculated the annual average TEEWRs (Table 1) of each river reach based on the calculated BEEWR of each section, the annual average river loss (${\overline{X}}_{\mathrm{l}\mathrm{o}\mathrm{s}\mathrm{s}}$), and the ecological water requirement for vegetation in dry areas (Y_ecology).

It has been assumed that the difference between the annual average runoff $(\overline{X})$ and X_{irrigation and domestic use} was used to meet ecological water requirements of natural vegetation, then, the calculation of x_loss (the corresponding river loss in the whole river for Y_ecology): (3) $$x_{\mathrm{l}\mathrm{o}\mathrm{s}\mathrm{s}}={\overline{X}}_{\mathrm{l}\mathrm{o}\mathrm{s}\mathrm{s}}·Y_{\mathrm{e}\mathrm{c}\mathrm{o}\mathrm{l}\mathrm{o}\mathrm{g}\mathrm{y}}/(\overline{X}-X_{\mathrm{i}\mathrm{r}\mathrm{r}\mathrm{i}\mathrm{g}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}\mathrm{a}\mathrm{n}\mathrm{d}\mathrm{d}\mathrm{o}\mathrm{m}\mathrm{e}\mathrm{s}\mathrm{t}\mathrm{i}\mathrm{c}\mathrm{u}\mathrm{s}\mathrm{e}})$$ where Y_ecology is the ecological water demand of vegetation calculated by the Diving Evaporation Method; ${\overline{X}}_{\mathrm{l}\mathrm{o}\mathrm{s}\mathrm{s}}$ is the annual average river loss along the entire river, as derived from Xinjiang Tarim River Basin Kashi Management Bureau; $\overline{X}$ is the annual average river runoff along the Kaqun reach (derived from the Xinjiang Tarim River Basin Kashi Management Bureau); and X_{irrigation and domestic use} is the amount of water diversion for irrigation and domestic use as determined by the Three Red Lines standard (the latter from the Xinjiang Tarim River Basin Kashi Management Bureau).

Based on the proportion between ${\overline{X}}_{\mathrm{l}\mathrm{o}\mathrm{s}\mathrm{s}}$ in the whole river and river losses (${\overline{x}}_{\mathrm{l}\mathrm{o}\mathrm{s}\mathrm{s}}$) along each river reach, the water loss along each river reach (x_{ecology–loss}) was determined from x_loss. Then, the different Y_ecology was reversed to obtain TEEWR for each section. Finally, the TEEWRs of the Kaqun reach were obtained as follows: (4) $$\mathrm{T}\mathrm{E}\mathrm{E}\mathrm{W}\mathrm{R}=Y_{\mathrm{e}\mathrm{c}\mathrm{o}\mathrm{l}\mathrm{o}\mathrm{g}\mathrm{y}}+\left({\overline{x}}_{\mathrm{l}\mathrm{o}\mathrm{s}\mathrm{s}}·x_{\mathrm{l}o\mathrm{s}\mathrm{s}}\right)/{\overline{X}}_{\mathrm{l}\mathrm{o}\mathrm{s}\mathrm{s}}$$

Target EEWR did not change with the amount of water in upstream reaches. Therefore, TEEWRs in each reach for the examined total annual flow frequencies were the same as the annual average TEEWRs.

Determination of the runoff along the Kaqun reach

A Log-Pearson Type III Frequency curve (P-III Frequency curve) was constructed for the Kaqun reach using runoff data from 1970 to 2017 (Fig. 2). Following the development of the curve, runoff data for different flow frequencies (25%, 50%, 75%, and 90%) were obtained, of which 25% are years of abundant water, 50% are flat years, and 75% and 90% are dry years and extremely dry years, respectively.

The river loss along each river reach

The formulas for determining x_{n loss} along each river reach for flows of a given return frequency were calculated as follows: (5) $${\overline{x}}_{n\mathrm{l}\mathrm{o}\mathrm{s}\mathrm{s}}={\displaystyle \frac{{\overline{X}}_{\mathrm{l}\mathrm{o}\mathrm{s}\mathrm{s}}}{\overline{X}}·x_n(n:25\mathrm{\%},50\mathrm{\%},75\mathrm{\%},90\mathrm{\%})}$$ where ${\overline{x}}_{n\mathrm{l}\mathrm{o}\mathrm{s}\mathrm{s}}$ is the river loss along the entire river for a given flow frequency; n is the flow frequency being considered; $\overline{X}$ is the annual average runoff at the Kaqun reach monitoring station (i.e., the runoff under the 50% flow frequency); ${\overline{X}}_{\mathrm{l}\mathrm{o}\mathrm{s}\mathrm{s}}$ is the annual average river loss along the whole river (that is the river loss under the 50% flow frequency); and x_n is the runoff along the Kaqun reach for a given flow frequency.

After ${\overline{x}}_{n\mathrm{l}\mathrm{o}\mathrm{s}\mathrm{s}}$ was calculated, the river loss along each river reach for each of the defined flow frequencies (x_{n loss}) was calculated using: (6) $$x_{n\mathrm{l}\mathrm{o}\mathrm{s}\mathrm{s}}={\displaystyle \frac{x_{50\mathrm{\%}\mathrm{l}\mathrm{o}\mathrm{s}\mathrm{s}}}{{\overline{x}}_{50\mathrm{\%}\mathrm{l}\mathrm{o}\mathrm{s}\mathrm{s}}}·100\mathrm{\%}·{\overline{x}}_{n\mathrm{l}\mathrm{o}\mathrm{s}\mathrm{s}}=A·{\overline{x}}_{n\mathrm{l}\mathrm{o}\mathrm{s}\mathrm{s}}}$$ where ${\overline{x}}_{50\mathrm{\%}\mathrm{l}\mathrm{o}\mathrm{s}\mathrm{s}}$ is the river loss along the whole river for the 50% flow frequency; x_{50% loss} is the river loss along each river reach for the 50% flow frequency; ${\overline{x}}_{n\mathrm{l}\mathrm{o}\mathrm{s}\mathrm{s}}$ is the river loss along the whole river for a specific flow frequency; x_{n loss} is the river loss along each river reach for each flow water frequency; and $A$ is the ratio between $x_{50\mathrm{\%}\mathrm{l}\mathrm{o}\mathrm{s}\mathrm{s}}$ and ${\overline{x}}_{50\mathrm{\%}\mathrm{l}\mathrm{o}\mathrm{s}\mathrm{s}}$, that is, 18%, 39%, 15%, and 27% in Kaqun-Yiganqi, Yiganqi-48th Regiment Ferry, 48th Regiment Ferry-Alektamu, and Alektamu-Heiniyazi, respectively.

In this paper, $\overline{X}$ = 68.16 × 10⁸ m³ (derived from Xinjiang Tarim River Basin Kashi Management Bureau), ${\overline{X}}_{\mathrm{l}\mathrm{o}\mathrm{s}\mathrm{s}}$ = 15.07 × 10⁸ m³ (derived from Xinjiang Tarim River Basin Kashi Management Bureau), x_n = 80.32 × 10⁸ m³, 71.78 × 10⁸ m³, 64.58 × 10⁸ m³, and 57.70 × 10⁸ m³ for the 25%, 50%, 75% and 90% flow frequencies, respectively (obtained by analyzing the P-III frequency curve), and ${\overline{x}}_{50\mathrm{\%}\mathrm{l}\mathrm{o}\mathrm{s}\mathrm{s}}$ = ${\overline{X}}_{\mathrm{l}\mathrm{o}\mathrm{s}\mathrm{s}}$ = 15.07 × 10⁸ m³ (derived from Xinjiang Tarim River Basin Kashi Management Bureau).

The amount of water (X_o) along other river reaches

Combined with the runoff (x_n) from the Kaqun reach for the different flow frequencies, the water diversion (X_{irrigation and domestic use}) and withdrawal (X_w) of water from each river reach, the loss of the interval river (x_loss), and the amount of water (X_o) from other study reaches can be obtained by the following formula: (7) $$X_o=X_p-x_{n\mathrm{l}\mathrm{o}\mathrm{s}\mathrm{s}}-X_{\mathrm{i}\mathrm{r}\mathrm{r}\mathrm{i}\mathrm{g}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}\mathrm{a}\mathrm{n}\mathrm{d}\mathrm{d}\mathrm{o}\mathrm{m}\mathrm{e}\mathrm{s}\mathrm{t}\mathrm{i}\mathrm{c}\mathrm{u}\mathrm{s}\mathrm{e}}+X_w$$ where X_o is the amount of water along one reach; X_p is the amount of water in a previous section; x_{n loss} is the interval river loss for the different flow frequencies; X_{irrigation and domestic use} is the “Three Red Lines” water diversion; and X_w is the water withdrawal. The “Three Red Lines” water plan for 2018 was used for X_{irrigation and domestic use} for the different flow frequencies.

Guaranteeing EEWRs for the different flow frequencies

The degree to which runoff along the different reaches met the BEEWR and TEEWRs was determined by calculating Water Requirement Index (WRI). The specific calculation method is: WRI = (X_o/BEEWR or TEEWR) × 100%. If WRI >100%, the amount of water at the monitoring station cross section is high for BEEWR or TEEWRs. If WRI < 100%, the amount of water at the cross section is low for BEEWR or TEEWRs.

For the determination of WRI of BEEWR, the runoff for the different flow frequencies was directly compared with BEEWR, and the calculation of WRI was obtained. After comparison with the first, second and third TEEWRs, WRI was calculated. Overall, for a given flow frequency, when WRI was less than 100% along a given reach, that frequency of flow could not meet the TEEWR.