**2. The proposed concept of "groundwater benefit zone"**

#### **2.1 Mechanism of water transport in salt-affected farmland**

Recently, numerous researches have been done on the water flow process and mechanism in soil–plant-atmosphere continuous (SPAC) systems. However, these studies do not fully consider the role of groundwater and cannot clarify the water transfer mechanism in groundwater-soil–plant-atmosphere continuum (GSPAC) systems. In particular, in saline groundwater areas, water utilization of crop is limited because of salt stress, and it seems impossible to determine how groundwater recharge the root zone nor its contribution to soil evaporation and crop transpiration [10]. In drought years, plants increase net primary productivity (NPP) by using groundwater to reduce the effect of water stress on CO2 fixation, resulting in significant increases in transpiration due to the presence of shallow groundwater. Lowry and Loheide [11] defined the additional water that the plant transpires from shallow groundwater as "groundwater subsidies", and calculated the difference of the root water absorption under shallow groundwater and the free drainage conditions. Furthermore, Zipper et al. [4] defined the yield from this additional water as a "groundwater yield subsidy". In agricultural systems, yield is usually more relevant to total water consumption when characterizing groundwater's positive or negative effects. Therefore, by introducing the concept of "groundwater yield subsidy", the maximum annual contribution of groundwater to transpiration and NPP can be quantified and directly related to the efficiency of water utilization.

On the contrary, when shallow groundwater damages production through oxygen stress, the groundwater yield subsidy is negative and can be considered a loss of groundwater yield. Soylu et al. [12] quantified annual groundwater subsidies and NPP changes using the AgroIBIS-VSF model. They found that the largest groundwater subsidy happens at 1.5–2 m of water table depth, regardless of longterm precipitation, described here as the optimal water table. However, the current AgroIBIS-VSF model study is carried out in the non-saline area, and the applicability of these indicators in saline-alkali land and its conceptual extension still needs to be further studied.

#### **2.2 Definition of the "groundwater benefit zone"**

In general, to prevent soil salinization, groundwater must be kept below the critical groundwater table [10, 13]. The scientific community currently lacks a recognized definition and quantification method for the critical groundwater table. We define it here as the highest groundwater table that does not cause secondary soil salinization. The critical water table depends on soil and groundwater type and climatic evaporation potential and is also related to the classification criteria for salinization. Theoretically, there is usually an optimal groundwater table in an agricultural ecosystem, ideal for maintaining farmland productivity. However, due

*The "Groundwater Benefit Zone", Proposals, Contributions and New Scientific Issues DOI: http://dx.doi.org/10.5772/intechopen.100299*

#### **Figure 1.**

*Diagram of crop-groundwater feed-in relationship in shallow groundwater area: (a) the hypothetical relationship between shallow groundwater level and crop (in the case of maize) yield; (b) the conceptual diagram of the groundwater benefit zone. Refer to Zipper et al. [4].*

to the complex factors which influence groundwater, it is often difficult to quantify. **Figure 1a** shows a conceptual diagram of the relationship between groundwater and crop yield under the groundwater yield subsidy framework: (1) In dry years, shallow groundwater will provide groundwater yield subsidy by reducing water stress, while in wet years, it will result in loss of groundwater yield by increasing oxygen stress; (2) In other words, for coarse soils with low matric potential values, the roots must be relatively close to the water table in case groundwater yield subsidies are present.

Theoretically, depending on the objectives of regulation, groundwater control has two criteria (**Figure 1b**):

1. It is necessary to control the groundwater table below its critical value to control the salinity of soil [10]; the critical groundwater table (*h*0) can be calculated by soil evaporation based on the upward migration of groundwater (*E*):

$$E = \begin{cases} E\_p \left( \mathbf{1} - \frac{h}{h\_0} \right)^n \left( \mathbf{1} - \frac{\rho - \rho\_r}{\rho\_0 - \rho\_r} \right), h < h\_0 \text{ and } \rho < \rho\_0 \\\ 0, h < h\_0 \text{ and } \rho \ge \rho\_0 \\\ 0, h \ge h\_0 \end{cases} \tag{1}$$

Where, *Ep* is the potential evaporation, *h* is the groundwater table, the *φ* is the electrical conductivity, *φ*<sup>0</sup> is the electrical conductivity corresponding to the critical water table, *φ<sup>r</sup>* is the threshold for salt stress, *n* is the parameter;

2. It is also necessary to keep the groundwater table close to the optimal groundwater table (groundwater yield subsidy boundary) [4] to maximize crop transpiration, which can be calculated through groundwatersubsidy-based-transpiration (*T*):

$$T(h,\rho,z,t) = a(h,\rho,z,t)\beta(z,t)T\_p(z,t)\tag{2}$$

Where, *z* is the soil depth, *t* is the time, *α* is the water-salt stress function of the crop rooting zone with the influence of groundwater, which is usually considered in the model (e.g., HYDRUS) as the product of the water stress function (*αh*) and the salt stress function (*αφ*). The stress function can be calculated by the following formula:

$$a\_h = \begin{cases} 0, & h \ge h\_{\max}, h \le h\_{\min} \\ \frac{h\_{\max} - h}{h\_{\max} - h\_c}, & h\_c < h < h\_{\max} \\ 1, & h = h\_c \\ \frac{h - h\_{\min}}{h\_c - h\_{\min}}, & h\_{\min} < h < h\_c \\ \frac{1}{1 + \left(\frac{h\_o}{h\_{\text{eq0}}}\right)^p} \end{cases} \tag{3}$$

Where, *hc*, *hmax*, *hmin* is the optimal water table and its maximum and minimum groundwater subsidy boundaries respectively, *h<sup>φ</sup>* is solute potential, and *hφ*<sup>50</sup> is the solute potential when the stress in the Van Genuchten salt stress function reduces the water absorption rate by 50%, p is related parameter.

In Eq. (2),*Tp* is the potential transpiration in the root region *β*, which together with *EP* in the Eq. (1) constitute the potential evapotranspiration in the field, can be calculated by the following formula:

$$\mathbf{E\_p(t)} = \mathbf{E} \mathbf{T\_p(t)} \cdot \mathbf{exp}^{-\mathbf{k} \cdot \text{LAl(t)}}$$

$$T\_p(t) = E T\_p(t) - E\_p(t) \tag{4}$$

Where, *ETp* is the potential evapotranspiration, which is usually calculated using the Penman-Monteith formula, *k* is the extinction coefficient, and LAI is the leaf area index.

Based on the equation above: (1) While critical groundwater table is an indicator to prevent soil salinization, the optimum groundwater table is an indicator to maximize groundwater subsidies, (2) The optimum groundwater table is an

*The "Groundwater Benefit Zone", Proposals, Contributions and New Scientific Issues DOI: http://dx.doi.org/10.5772/intechopen.100299*

agrological parameter based on the water absorption by the root system, whereas the critical groundwater table is a hydrological parameter based on soil capillary theory; (3) The critical groundwater table associated with soil salt content control, is a fixed value, while the groundwater table associated with groundwater yield subsidy is a range (which changes with the crop rooting pattern and the water-salt environment in the root zone). Although the effects of salinity on plants are also taken into account in some studies for defining the critical groundwater table (similar to the dynamic range of the groundwater table suitable for the crop), due to the complex coupling relationship between crop type, soil salinity, and groundwater depth, there is often a lack of quantitative indicators or appropriate methods to apply directly [13].

Consequently, in underground saltwater areas, if both soil salt control and groundwater subsidies are taken into account, the water table needs to be regulated below the critical water table and overlapping with the area of the range of groundwater yield subsidies (as shown in **Figure 2** yellow plus area), which we define as the "groundwater benefit zone" (Δ*h*), mathematically expressed as:

$$\Delta h = \begin{cases} 0, h\_0 < h\_{\min} \\ h\_0 - h\_{\min}, h\_{\min} \le h\_0 \le h\_{\max} \\ h\_{\max} - h\_{\min}, h\_0 > h\_{\max} \end{cases} \tag{5}$$

Therefore, the groundwater benefit zone proposed in this study is a newly defined index. Take it as the theoretical standard of groundwater regulation, it is easy to create the targeted groundwater level and adjust the groundwater level by taking specific control measures. It should be emphasized that, similar to critical and optimal groundwater tables, which define only the characteristics of water levels in vertical directions, the groundwater benefit zone defined by this study is also limited to vertical directions, regardless of their changes in horizontal direction (**Figure 2**).

**Figure 2.** *Schematic diagram of definition of groundwater benefit zone.*

To sum up, the physical significance of the "groundwater benefit zone" index defined in this study is clear, which can be used to quantify the potential of groundwater's contribution to the productivity of farmland ecosystem under the condition of salt stress and also as the theoretical standard of groundwater regulation in GSPAC system.
