Interaction of soil water and groundwater during the freezing-thawing cycle: field observations and numerical modeling

Freezing-induced water migration and groundwater level decline are widely observed in regions with shallow water table, but many existing studies trying to quantify freezing-induced groundwater migration do not account for water level fluctuations induced by freezing and thawing. Here, detailed field observations of liquid soil water content and groundwater level fluctuations at a site in the Ordos Plateau, China are combined with numerical modeling to show groundwater and soil water dynamics controlled by wintertime atmospheric conditions and topographically-driven lateral groundwater inflow. By comparing simulation results with and without lateral groundwater inflow, we find lateral groundwater inflow leads to an alleviated freezing-induced water level decline and enhanced freezing-induced water migration. At the field site with a lateral groundwater inflow rate of 1.03 mm/d, compared with the case without lateral groundwater inflow, the water level decline decreases from 40 cm to 15 cm, and the increased total water content in the frozen zone enhances from 0.071 to 0.106. By calculating the budget of groundwater, the mean upward flux of freezing-induced groundwater loss is 1.46 mm/d for 93 days, and the mean flux of thawing-induced groundwater gain is as high as 3.94 mm/d for 32 days. The study enhances our understanding of the mechanisms controlling water redistribution between saturated and unsaturated zones and the water budget in the freezing-thawing cycle. The fluxes of groundwater loss and gain in the freezing and thawing stages obtained in the current study can be useful for future studies on two- or three-dimensional transient groundwater flow in semi-arid regions with seasonally frozen soils.

Water Level Decline in a Reservoir: Implications for Water Quality Variation and Pollution Source Identification

Continuous water-level decline makes the changes of water quality in reservoirs more complicated. This paper uses trend analyses, wavelet analysis and principal component analysis-multiple linear regression to explore the changes and pollution sources affecting water quality during a period of continuous reservoir water level decline (from 65.37 m to 54.15 m), taking the Biliuhe reservoir as an example. The results showed that the change of water level of Biliuhe reservoir has a significant 13-year periodicity. The unusual water quality changes during the low water level period were as follows: total nitrogen continued to decrease. And iron was lower than its historical level. pH, total phosphorus, and ammonia nitrogen were higher than historical levels and fluctuated seasonally. Permanganate index increased as water level decreased after initial fluctuations. Dissolved oxygen was characterized by high content in winter and relatively low content in summer. The pollutant sources of non-point source pollution (PC1), sediment and groundwater pollution (PC2), atmospheric and production & domestic sewage (PC3), other sources of pollution (PC4) were identified. The main source of DO, pH, TP, TN, NH4-N, Fe and CODMn were respectively PC3 (42.13%), PC1 (47.67%), PC3 (47.62%), PC1 (29.75%), PC2 (47.01%), PC1 (56.97%) and PC2 (50%). It is concluded that the continuous decline of water level has a significant impact on the changes and pollution sources affecting water quality. Detailed experiments focusing on sediment pollution release flux, and biological action will be explored next.

Climate change or irrigated agriculture – what drives the water level decline of Lake Urmia?

<p>Lake Urmia is one of the largest hypersaline lakes on earth with a unique biodiversity. Over the past two decades the lake water level declined dramatically, threatening the functionality of the lake’s ecosystems. There is a controversial debate about the reasons for this decline, with either mismanagement of the water resources, or climatic changes assumed to be the main cause.</p><p>During this study we gathered an extensive hydro-meteorological data set, information about the reservoirs and the lake bathymetry. This data served for a quantification of the water budget components of Lake Urmia over the last five decades. Interestingly, a comparison of the temporal patterns of the principal natural boundary conditions of streamflow (precipitation and evaporation) with the inflow to the lake revealed that the variability of the inflow can be well explained its natural drivers. With this we can show that variations of Lake Urmia’s water level during the analyzed period were mainly triggered by climatic changes.</p><p>However, under the current climatic conditions agricultural water extraction volumes are significant and often exceed the remaining surface water inflow volumes. This rather simple observation shows that something deeper needs to be dug here. Therefore, we performed a parsimonious hindcast experiment and run a set of development scenarios based on the previously developed water balance. This helped us to better quantify the human impact on the development of the water volume of Lake Urmia. We could show that changes in agricultural water withdrawal would have a significant impact on the lake volume and could either stabilize the lake, or lead to its complete collapse (Schulz et al., 2020).</p><p> </p><p><strong>References</strong></p><p>Schulz, S., Darehshouri, S., Hassanzadeh, E., Tajrishy, M. and Schüth, C.: Climate change or irrigated agriculture – what drives the water level decline of Lake Urmia, Sci. Rep., 10(1), 236, doi:10.1038/s41598-019-57150-y, 2020.</p>

Estimation of shallow groundwater evaporation from dried-up areas of Lake Urmia, Iran

<p>Lake Urmia, located in the northwest of Iran, had an initial volume of about 19 km<sup>3</sup> and a surface area of 5,700 km<sup>2</sup> (Alipour, 2006). Once one of the largest hypersaline lakes in the world, this UNESCO Biosphere Reserve site currently shows a remarkable water level decline. About 70% of the lake area (Tourian et al., 2015) and more than 90% of its volume were lost between 2000 and 2014 (Schulz et al., 2020). The lack of a precise water balance of the Lake Urmia catchment is one of the challenges authorities are facing in their efforts to restore the lake to its ecological level. Here, key issues are that lake evaporation rates are mostly assumed and that evaporation of shallow groundwater from dried-up areas (up to 3,000 km<sup>2</sup>) is often ignored. The objective of this study is to obtain evaporation rate estimates for the dried-up parts of the Urmia lake bed. To this end, we set up a laboratory experiment with undisturbed soil columns collected from dried-up areas of the lake. With the help of a custom-made low-cost environmental chamber, the columns were subject to day- and night-time weather conditions typical for the area. Performed measurements comprise water level logging and monitoring of mass losses from the columns due to evaporation. First experimental results will be presented.</p><p> </p><p><strong>References </strong></p><p>Alipour, S., 2006. Hydrogeochemistry of seasonal variation of Urmia Salt Lake, Iran. Saline Systems 2, 9. doi:10.1186/1746-1448-2-9</p><p>Schulz, S., Darehshouri, S., Hassanzadeh, E., Tajrishy, M., Schüth, C., 2020. Climate change or irrigated agriculture – what drives the water level decline of Lake Urmia. Sci. Rep. 1–10. doi:10.1038/s41598-019-57150-y</p><p>Tourian, M.J., Elmi, O., Chen, Q., Devaraju, B., Roohi, S., Sneeuw, N., 2015. A spaceborne multisensor approach to monitor the desiccation of Lake Urmia in Iran. Remote Sens. Environ. 156, 349–360. doi:10.1016/j.rse.2014.10.006</p><p> </p>