Partitioning a regional groundwater flow system into shallow local and deep regional flow compartments

International audienceThe distribution of groundwater fluxes in aquifers is strongly influenced by topography, and organized between hillslope and regional scales. The objective of this study is to provide new insights regarding the compartmentalization of aquifers at the regional scale and the partitioning of recharge between shallow/local and deep/regional groundwater transfers. A finite-difference flow model was implemented, and the flow structure was analyzed as a function of recharge (from 20 to 500 mm/yr), at the regional-scale (1400 km2), in three dimensions, and accounting for variable groundwater discharge zones; aspects which are usually not considered simultaneously in previous studies. The model allows visualizing 3-D circulations, as those provided by Tothian models in 2-D, and shows local and regional transfers, with 3-D effects. The probability density function of transit times clearly shows two different parts, interpreted using a two-compartment model, and related to regional groundwater transfers and local groundwater transfers. The role of recharge on the size and nature of the flow regimes, including groundwater pathways, transit time distributions, and volumes associated to the two compartments, have been investigated. Results show that topography control on the water table and groundwater compartmentalization varies with the recharge rate applied. When recharge decreases, the absolute value of flow associated to the regional compartment decreases, whereas its relative value increases. The volume associated to the regional compartment is calculated from the exponential part of the two-compartment model, and is nearly insensitive to the total recharge fluctuations

A Channel Network Evolution Model with Subsurface Saturation Mechanism and Analysis of the Chaotic Behavior of the Model

Prepared under the support of the National Science Foundation, Army Research Office and National Research Council of Italy

Power-law distributions of mass and energy in river basins.

River networks constitute dissipative systems with many spatial degrees of freedom. Previous work by Mandelbrot (1983) and Bak et al. (1987, 1988, 1990) suggests that such systems will follow power law distributions in their mass and energy characteristics. It is shown that this is the case for river networks where the exponent β in the distribution, P[X \u3e x] ∝ x−β, is approximately equal to 0.45 and 0.90 for discharge and energy respectively in the case of several networks analyzed in North America when these variables are calculated for each individual link throughout the drainage network. An explanation of the values of β is offered based on the fractal structure of rivers and on principles of energy expenditure in river basins

The topographic signature of a major typhoon

none4siIn August 2009, the typhoon Morakot, characterized by a cumulative rainfall up to 2884 mm in about three days, triggered thousands of landslides in Taiwan. The availability of LiDAR surveys before (2005) and after (2010) this event offers a unique opportunity to investigate the topographic signatures of a major typhoon. The analysis considers the comparison of slope–area relationships derived by LiDAR digital terrain models (DTMs). This approach has been successfully used to distinguish hillslope from channelized processes, as a basis to develop landscape evolution models and theories, and understand the linkages between landscape morphology and tectonics, climate, and geology. We considered six catchments affected by a different degree of erosion: three affected by shallow and deep-seated landslides, and three not affected by erosion. For each of these catchments, 2 m DTMs were derived from LiDAR data. The scaling regimes of local slope versus drainage area suggested that for the catchments affected by landslides: (i) the hillslope-to-valley transitions morphology, for a given value of drainage area, is shifted towards higher value of slopes, thus indicating a likely migration of the channelized processes and erosion toward the catchment boundary (the catchment head becomes steeper because of erosion); (ii) the topographic gradient along valley profiles tends to decrease progressively (the valley profile becomes gentler because of sediment deposition after the typhoon). The catchments without any landslides present a statistically indistinguishable slope–area scaling regime. These results are interesting since for the first time, using multi-temporal high-resolution topography derived by LiDAR, we demonstrated that a single climate event is able to cause significant major geomorphic changes on the landscape, detectable using slope–area scaling analysis. This provides new insights about landscape evolution under major climate forcing.restrictedC. Tseng; C. Lin; G. Dalla Fontana; P. TarolliC., Tseng; C., Lin; DALLA FONTANA, Giancarlo; Tarolli, Paol