Study on extinction depth and steady water storage in root zone based on lysimeter experiment and HYDRUS-1D simulation

In areas where the water table is within or slightly below the root zone, crops uptake water from both a thin unsaturated vadose zone and saturated ground water (Shah et al. 2007). Groundwater contribution to crop water use by groundwater evaporation (ET_gw) is one important term of soil water balance in the presence of shallow water tables (Liu et al. 2006; Bourgault et al. 2014). Therefore, groundwater should be used efficiently (Boyraz & Kazezyilmaz-Alhan 2014). The volume of drainage and irrigation water can be reduced by using shallow groundwater to meet a portion of the crop water requirement (Ayars et al. 1999). However, the groundwater contribution is influenced by many factors, and among them, groundwater depth and the soil water storage in root zone are two important influencing factors of ET_gw (Raes & Deproost 2003; Doble et al. 2006; Luo & He 2008).

ET_gw was often assumed to have a linear or piecewise linear relationship with groundwater table depth (McDonald & Harbaugh 1988; Banta 2000; Schmid et al. 2006) and vanished at the depth which is termed extinction depth, which depends on the types of soil or vegetative cover. Vegetation types may have significant influences on soil water flux (Moiwo & Tao 2014). Based on HYDRUS simulation, Shah et al. (2007) gave the extinction depths for three land covers: bared soil, shallow-rooted (grass) and deep-rooted vegetation (forest). However, for the same type of land covers, large differences in the maximum root depth and extinction depth may occur for different vegetation. The values recommended by Shah et al. (2007) are approximate, but may not be exact in application.

Liu & Luo (2012) proposed an improved approach determining ET_gw-max based on the approaches of Food and Agriculture Organization of the United Nations Paper No. 24 (FAO-24) (Doorenbos & Pruitt 1977) and FMP1 (Farm Process) (Schmid 2006). Askri et al. (2010) replaced W_p with the storage at capacity (W_fc) because ET_gw became negligible when the actual storage was above W_fc. Liu et al. (2006) assumed that a steady-state occurred in the soil profile and ET_gw occurred at the potential rate (ET_gw-max) when the actual soil water storage was lower than a steady storage (W_s), and replaced lower storage (W_wp) in Equation (1) with W_s. Liu & Luo (2012) found that the water storage in the 0–60 cm soil layer under winter wheat and silt soil trended to be a relatively steady value by the measured data through lysimeter experiments.

Many researchers have studied ET_gw under crop planting with lysimeters (Hutmacher et al. 1996; Kahlown et al. 2005; Ayars et al. 2009; Huo et al. 2012). However, little experimental study has been conducted to investigate intensively the impact of soil water storage on ET_gw. Moreover, because the application of a lysimeter is limited by the cost in construction, operation and maintenance (Babajimopoulos et al. 2007), it is difficult to have enough experiment treatments to determine the extinction depth and W_s of a wide range of groundwater depths. The combination of lysimeter and numerical model provides an efficient method to determine these values.

HYDRUS-1D was developed to solve the Richards equation and widely used to simulate one-dimensional water movement in variably saturated media (Simunek et al. 2008) and was applied widely to study soil water movement and root-water uptake (Ma et al. 2010; Shouse et al. 2011; Leterme et al. 2012; Cheng et al. 2013; Zhu et al. 2013). The objective of this study was to determine the extinction depth and W_s for a wide range of groundwater depths by the combination of lysimeter experiments and HYDRUS-1D simulation under winter wheat (Triticum aestivum L.) and silt soil.

Field experiment was conducted at the Yucheng Comprehensive Experimental Station of the Chinese Academy of Sciences (116°36′E and 36°57′N) at Yucheng City, Shandong Province, China. It is under a semi-humid and semi-arid climate, and the annual mean temperature and precipitation are 13.1 °C and 600 mm, respectively. The precipitation during the growth period of winter wheat is about 150 mm, which is far less than its water requirement. However, the water table depth mainly fluctuates between 0.5 and 3.0 m due to the recharge of precipitation and irrigation (Luo & He 2008). The contribution of shallow groundwater to the crop water requirement may help to save irrigation water.

The wilting point and field capacity of the original silt soil in the soil container were 0.07 and 0.32 cm³ cm⁻³. The bulk density and mechanical composition of the soil are shown in Table 1.

The experiments were conducted in 2004–2005 and 2007–2008, and the treatments in the two growth seasons were denoted as ‘T1’ and ‘T2’, respectively. The planting crop was winter wheat (T. aestivum L.). The field management and the development of the growth stage of winter wheat are shown in Tables 2 and 3. Three constant groundwater depths were 0.7, 1.1 and 1.5 m in the period from 23 March to 8 June 2005 during which the treatments received precipitation (Table 4). There was only one constant groundwater depth, 1.5 m, in the period from 20 March to 5 June 2008 during which the treatment did not receive precipitation. To quantify the ET_gw under the condition without irrigation, no irrigation was applied for all experiment treatments in both growth seasons.

Daily ET_gw was measured through lysimeters. Soil volumetric water content was measured with neutron probes (CNC503B, China) at five to seven intervals and in 0.1 m increments from 0.1 to 1.3 m deep into the soil profile. Meteorological data such as precipitation and wind speed were measured in a standard weather station 50 m away from the lysimeters.

Based on the mechanical composition and bulk density of the soil, the soil hydraulic parameters such as θ_r, θ_s, α, n and K_s were determined first by using the neural network prediction function in HYDRUS-1D. The measured data including the soil water storage in the 0–60 cm layer (the main distribution zone of the root system for winter wheat) and daily ET_gw in 2005 were used to manually calibrate the parameters, and the measured data in 2008 were used for validation. The calibrated parameters are shown in Table 5.

To determine W_s and the extinction depth for a wide range of groundwater depths, 13 groundwater depths from 0.5 to 5.0 m were set up for scenario simulation. The increment was 0.2 m from 0.5 to 1.5 m (i.e. 0.5, 0.7, 0.9, 1.1, 1.3 and 1.5 m), and 0.5 m from 2.0 to 5.0 m (i.e. 2.0, 2.5, 3.0, 3.5, 4.0, 4.5 and 5.0 m). The simulation period was from 26 March to 8 June 2005, and the measured meteorological data in the corresponding period were used but no precipitation was considered for obtaining and keeping steady soil water distribution.

The steady soil water content was obtained by the following steps. (1) The initial soil water content above the water table was assumed to be the field capacity (0.32 cm³ cm⁻³), and the soil water profile was simulated for each day from 26 March to 8 June 2005. (2) The soil water profile in the last day of the first simulation period (8 June 2005) was adopted to be the initial profile of the second round of simulation (26 March 2005). (3) This process would not stop until the soil water content distribution differences in the last days of two adjacent simulation periods was negligible. The soil water storage from 0 to 60 cm depth of the last simulation was assumed to be the steady storage in the root zone for each scenario.

There was also a good agreement between the measured and simulated soil water storages in the 0–60 cm layer for the validation period (Figure 2(b) and Table 6). The RE and RMSE were −2.6% and 6.91 mm, respectively. As shown in Figure 3(d), the ET_gw was overestimated around 20 April 2008 due to the influence of precipitation. The precipitation amount was 34.2 mm on 20 April 2008, and the treatments received precipitation due to operation delay at night. However, the general trends between the measured and simulated ET_gw were similar, and the RE and RMSE were only 6.0% and 0.67 mm d⁻¹, respectively. Therefore, it can be concluded that the simulation of ET_gw using HYDRUS-1D is reliable.

It should be noted that the daily ET_gw values were overestimated for the late season stages (30 May to 8 June 2005 and 1–5 June 2008). In the late season stage, the crop was harvested and dried out (Allen et al. 1998), and the consumption of groundwater water by winter wheat also declined. However, the model did not reflect this change well. The recommended basal crop coefficient (that was 0.3 in this study) in the late season stage (K_cbend) (Allen et al. 1998) might be large and hence result in the overestimation of ET_gw.

Figure 4 shows that the average daily ET_gw was higher than 1.49 mm d⁻¹ when groundwater depth was in the range from 0.5 to 2.5 m and decreased rapidly with increasing groundwater depth. When groundwater depth was in the range from 2.5 to 5.0 m, the average daily ET_gw was lower than 1.49 mm d⁻¹ and decreased slowly with increasing groundwater depth. ET_gw was lower than 0.20 mm d⁻¹ when groundwater depth was at 5.0 m (0.19 mm d⁻¹). Under the condition of silt soil and winter wheat, the extinction depth may be determined to be 5.0 m. Shah et al. (2007) found a similar result that the extinction depth was 5.3 m under the condition of silt soil and grass.

HYDRUS-1D simulation was combined with lysimeter experiments to determine the extinction depth and W_s for a wide range of groundwater depths under winter wheat and silt soil. The measured soil water storage in the 0–60 cm layer and daily ET_gw were used to calibrate and validate the parameters of the HYDRUS model. It showed that HYDRUS-1D simulation gave reliable results. The results showed that ET_gw decreased linearly with increasing groundwater depth from 0.5 to 2.5 m and decreased as a power function with increasing groundwater depth from 2.5 to 5.0 m. Under the condition of silt soil and winter wheat, the extinction depth of ET_gw was about 5.0 m. In addition, the results also showed that W_s decreased linearly with increasing groundwater depth ranging from 0.7 to 2.0 m and was not influenced by ground water depth larger than 2.0 m.

This research was partially financed by the Natural Sciences Foundation of China (No. 41301021) and the Project of Chinese Academy of Sciences (No. CXJQ120109).