RĂ©sultat de la recherche
Salix psammophila afforestations can cause a decline of the water table, prevent groundwater recharge and reduce effective infiltration
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.
Potential evaporation dynamics over saturated bare soil and an open waters surface
Actual evaporation (Ea) can be calculated as a fraction of potential evaporation (PE), which refers to the evaporation rate if supply water is unlimited. Potential evaporation depends on the available energy and the underlying material, and different approaches to estimate potential evaporation exist nowadays. This study provides a detailed analysis of the evaporation dynamics over fully saturated, sandy soil (PEs) and an open water surface (PEw). Moreover, the performance of commonly used methods to estimate PE is assessed. At the basis of these analyses is a lysimeter experiment in the Guanzhong Basin, China, which allowed a precise measurement of PE with a very high temporal resolution. Temperature profiles in lysimeters and meteorological data were also measured during the experiment. A comparison of PEs and PEw was carried out for seven consecutive days (August 11th to 17th, 2016). Results show that PEw is smaller than PEs on a daily scale, with PEw rates being bigger than PEs at night but smaller during daytime. Furthermore, the temporal dynamics of PEw lags 4–5 h behind PEs. In accordance with the energy balance equation, PE dynamics are mainly governed by “available energy”. The PE rates calculated by Penman-Monteith (PM) and Priestly-Taylor (PT) based on these measurements were also evaluated. The measured PE is relatively well reproduced by PM and PT equations. Finally, the effect of using different approaches to estimate PE on calculating Ea was tested by an integrated hydrological model that calculates water flow in the unsaturated zone by solving the Richards equation. The relative differences were up to 17.5%.
Assessing bare-soil evaporation from different water-table depths using lysimeters and a numerical model in the Ordos Basin, China
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.