Brunner, Philip

2021-8, Zhang, Zaiyong, Wang, Wenke, Gong, Chengcheng, Zhao, Ming, Hendricks Franssen, Harrie-Jan, Brunner, Philip

Afforestation can reduce desertification and soil erosion. However, the hydrologic implications of afforestation are not well investigated, especially in arid and semi-arid regions. China has the largest area of afforestation in the world, with one-third of the world's total plantation forests. How the shrubs affect evapotranspiration, soil moisture dynamics, and groundwater recharge remains unclear. We designed two pairs of lysimeters, one being 1.2 m deep and the other one 4.2 m deep. Each pair consists of one lysimeter with bare soil, while on the other one a shrub is planted. The different water table depths were implemented to understand how depth to groundwater affects soil moisture and water table dynamics under different hydrological conditions. Soil moisture, water table depth, sap flow, and rainfall were measured concurrently. Our study confirms that for the current meteorological conditions in the Ordos plateau recharge is reduced or even prohibited through the large-scale plantation Salix psammophila. Shrubs also raise the threshold of precipitation required to increase soil moisture of the surface ground. For the conditions we analyzed, a minimum of 6 mm of precipitation was required for infiltration processes to commence. In addition to the hydrological analysis, the density of root distribution is assessed outside of the lysimeters for different water table depths. The results suggest that the root-density distribution is strongly affected by water table depth. Our results have important implications for the determination of the optimal shrub-density in future plantations, as well as for the conceptualization of plant roots in upcoming numerical models.

2020-8-1, Gong, Chengcheng, Wang, Wenke, Zhang, Zaiyong, Wang, Hao, Luo, Jie, Brunner, Philip

Evaporation from bare soil is an important component of a catchment water balance. However, it is arguably one of the most challenging hydrological processes to estimate and measure accurately. Several approaches to estimate soil evaporation exist, but their performance for specific water table conditions remains unclear. This study investigated the performance of four commonly used approaches and several ways on how to implement them: the energy-balanced based FAO-56 method with the skin evaporation enhancement (FAO-56 skin), hydraulic methods based on groundwater level fluctuation (GLF), Darcy’s law, and the maximum entropy production (MEP) method based on non-equilibrium thermodynamics theory. Three lysimeters with different water table depths were used at a research site in the Guanzhong Basin of China. The lysimeters were equipped with soil moisture probes. Water table fluctuations were also measured. The data allow us to accurately estimate evaporation rates using a water balance approach and are used to assess the performance of the analysed methods. The results show that: (1) The MEP method performed best for all water table conditions, but tends to overestimate evaporation if the water table is below the extinction depth. The extinction depth is the depth of the water table were there the contribution of groundwater to bare-soil evaporation is zero. In our case, the extinction depth was 78 cm. (2) The FAO-56 skin method underestimated evaporation where the water table was above the extinction depth, and vice versa. (3) The groundwater level fluctuation method significantly overestimated the evaporation if the specific yield was estimated using hydraulic methods. The groundwater level fluctuation method should be combined with a soil water balance, independent of water table conditions. The method can only be applied if the water table is above the extinction depth. (4) Conceptually, Darcy’s law was suitable for estimating evaporation. However, the estimation of the required parameters is challenging. A good fit could only be obtained through calibration to measured evaporation rates.

2019-7, Ma, Zhitong, Wang, Wenke, Zhang, Zaiyong, Brunner, Philip, Wang, Zhoufeng, Chen, Li, Zhao, Ming, Gong, Chengcheng

In semiarid and arid regions, the evaporation from bare soil is highly sensitive to changes in the depth to the water table. This study quantifies the relation between water-table depth and the groundwater contribution to evaporation in the Ordos Basin in China. In-situ field experiments were combined with numerical simulations of heat, vapor and liquid water flow. Based on lysimeter experiments and a calibrated numerical model, a relation between depth to groundwater and evaporation rate was established for the lysimeter site. In addition, a sensitivity analysis considering the hydraulic conductivity and the inverse of the air-entry pressure (vanGenuchten α) was established. For the field site, the results showed that for the water-table depths less than 52 cm below the ground, evaporation is independent of the water-table depth. For water-table depths exceeding 52 cm, an exponential relation between depth to groundwater and evaporation is observed. No phreatic evaporation occurs for water tables deeper than 105 cm, which is nearly two times the capillary fringe height. The sensitivity analysis showed that the extinction depth decreased with decreasing hydraulic conductivity and increased with α. The field-specific results and the sensitivity analysis provide valuable information to understand the dynamic processes of soil evaporation in the Ordos Basin. From a methodological point of view, the proposed modelling approach and the integration of lysimeter data proved to be a highly efficient combination to study evaporation dynamics in semi-arid and arid environments.