Abstract
The focus of this paper is to investigate the ability to assess the flow exchanges between the matrix and the conduits in two karstified watersheds (Aliou and Baget, Ariège, France) using the KarstMod modeling platform. The modeling is applied using hourly and daily time series. First, the flow dynamics between the conduit and the surrounding matrix are described on a rainfall event scale (i.e., a few days). The model allows us to describe a physical reality concerning the flow reversal between matrix and conduit when there is a significant rainfall event. Then, the long-term trends (i.e., inter-annual) in the matrix water level are evidenced using the moving average over shifting horizon method (MASH). The mean water level in the matrix dropped about 10% to 15% since the late 1960s. Also, the matrix recharge has been delayed from February in the late 1960s to April since the 1990s. Moreover, the contribution of the matrix in the total spring flow is estimated though mass balance. It is estimated that the annual matrix contribution in the total spring flow is about 3% and it can increase to up to 25% during periods with low rainfall.
Highlights
Carbonate watersheds throughout the world are very often characterized by karst features.Karstification processes occur when the following conditions are met: (1) mechanical actions create fissures and fractures; (2) water-dissolved CO2 reacts with carbonate rock and (3) hydraulic gradient is sufficiently high to ensure the water renewal and to maintain the karstification potential [1]
The model is calibrated on both hourly and daily time series, allowing us to study the internal flow over different observations scale: (1) the hourly monitored time series allow a better description of the physical processes at the flood event scale and (2) the daily monitored time series measured over 50 years allow us to identify long-term trends in the internal fluxes and in the reservoirs water level
Considering the daily rainfall time series, the analysis shows that the mean rainfall during the past decades
Summary
Karstification processes occur when the following conditions are met: (1) mechanical actions create fissures and fractures; (2) water-dissolved CO2 reacts with carbonate rock and (3) hydraulic gradient is sufficiently high to ensure the water renewal and to maintain the karstification potential [1]. This results in complex structures with a heterogeneous permeability field and non-linear hydraulic behavior. Drainage in karstic systems consists of an underground self-organized structure leading to preferential water pathways through a conduit network embedded in either a porous matrix which sometimes is highly permeable (i.e., Floridan Karst aquifer) or a low permeable fissured matrix [2,3]. This drainage organization might be divided between (1) a ‘main drainage system’ (MDS), associated to rapid transit time and (2) an ‘annexe drainage system’ (ADS), associated with a long period of water residence [4,5].
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