Initial Water Table Research Articles

Numerical Model of the Low Temperature Geothermal Area for Direct use in Pohang, South Korea

We determined the horizontal boundary of a watershed in the study area for numerical simulation, in order to facilitate the direct use of geothermal water. The geothermal gradient of the research well is about 30.0°C/km and the measured temperature is 91°C at a depth of 2000 m. The initial water table is located at 360 m depth and 32.6 m drawdown was observed when the pumping rate was 400 m3/day over 55hours. Coupled fluid and heat transport simulations were conducted using the data of hydraulic tests and well inventory. In order to test the numerical model we used several pumping rates and a recharge rate. Simulated water levels at layers of the numerical model were evaluated from the hydrogeologic data of the study area.

Experimental study of airflow induced by pumping tests in unconfined aquifer with low-permeability cap

The airflow in unsaturated soils is an issue of great importance in various fields such as in agricultural, nuclear, environmental engineering. However, up to now, little attention was paid to the generation of the airflow induced by a pumping test in an unconfined aquifer with a low-permeability cap and its influence on the pumping test. In this paper, pumping tests were carried out experimentally in the aquifer with a low-permeability cap in order to study the influence of the airflow induced by the pumping on the drawdown of the tests. It is shown that: (1) there is an airflow with negative pressure generated by the pumping tests, (2) the Negative air Pressure (NP) is increased with the pumping rate but decreased with the radial distance, and (3) the NP also changes with the initial water table. The results provide a good basis for further theoretical study of the airflow induced by pumping.

Improving riparian wetland conditions based on infiltration and drainage behavior during and after controlled flooding

Summary We present results of an observational and modeling study of the hydrologic response of a riparian wetland to controlled flooding. The study site is located in Poopenaut Valley, Yosemite National Park (USA), adjacent to the Tuolumne River. This area is flooded periodically by releases from the Hetch Hetchy Reservoir, and was monitored during one flood sequence to assess the relative importance of inundation versus groundwater rise in establishing and maintaining riparian wetland conditions, defined on the basis of a minimum depth and duration of soil saturation, and to determine how restoration benefits might be achieved while reducing total flood discharge. Soil moisture data show how shallow soils were wetted by both inundation and a rising water table as the river hydrograph rose repeatedly during the controlled flood. The shallow groundwater aquifer under wetland areas responded quickly to conditions in the adjacent river, demonstrating a good connection between surface and subsurface regimes. The observed soil drainage response helped to calibrate a numerical model that was used to test scenarios for controlled flood releases. Modeling of this groundwater–wetland system suggests that inundation of surface soils is the most effective mechanism for developing wetland conditions, although an elevated water table helps to extend the duration of soil saturation. Achievement of wetland conditions can be achieved with a smaller total flood release, provided that repeated cycling of higher and lower river elevations is timed to benefit from the characteristic drainage behavior of wetland soils. These results are robust to modest variations in the initial water table elevation, as might result from wetter or dryer conditions prior to a flood. However, larger changes to initial water table elevation, as could be associated with long term climate change or drought conditions, would have a significant influence on wetland development. An ongoing controlled flooding program in Poopenaut Valley should help to distribute fine grained overbank deposits in wetland areas, extending the period of soil water retention in riparian soils.

응력 -간극수압 연계해석을 이용한 흙막이 굴착시 지하수저하에 따른 지반침하에 관한 연구

본 연구에서는 흙막이 굴착 공사시 지하수저하에 의한 주변 지반의 공학적 거동에 관한 수치해석 연구내용을 다루었다. 먼저 2차원 간극수압-응력 연계해석을 통하여 굴착공사가 지반침하 거동에 미지는 영향을 알아보았으며 그 결과를 토대로 벽체 수평변형, 벽체배변 변형, 소성변형률 분포, 유효응력분포 및 버팀보 축력분포 등을 분석을 수행하였다. 한편 다양한 지층조건과 초기 지하수위 조건에 대한 매개변수 연구를 통하여 주변지반 및 버팀보의 거동을 분석하였으며 해석 결과를 토대로 특정조건의 최대 지반변형범위를 제안하였다. This paper presents the results of a numerical investigation on the behavior of earth retaining wall system with emphasis on the groundwater lowering. Using the 2D stress-pore pressure coupled analysis, the effects of ground excavation and groundwater interaction were examined using wall horizontal deformation, ground surface movement, plastic strain pattern, effective stress distribution and axial stress of strut. In addition, based on the results from a parametric study on a wide range of soil profile and initial ground water table level, the ranges of wall displacement and ground deformation were suggested quantitatively.

Saturated‐unsaturated flow to a well with storage in a compressible unconfined aquifer

Mishra and Neuman (2010) developed an analytical solution for flow to a partially penetrating well of zero radius in a compressible unconfined aquifer that allows inferring its saturated and unsaturated hydraulic properties from responses recorded in the saturated and/or unsaturated zones. Their solution accounts for horizontal as well as vertical flows in each zone. It represents unsaturated zone constitutive properties in a manner that is at once mathematically tractable and sufficiently flexible to provide much improved fits to standard constitutive models. In this paper we extend the solution of Mishra and Neuman [2010] to the case of a finite diameter pumping well with storage; investigate the effects of storage in the pumping well and delayed piezometer response on drawdowns in the saturated and unsaturated zones as functions of position and time; validate our solution against numerical simulations of drawdown in a synthetic aquifer having unsaturated properties described by the van Genuchten [1980]–Mualem [1976] model; use our solution to analyze 11 transducer‐measured drawdown records from a seven‐day pumping test conducted by University of Waterloo researchers at the Canadian Forces Base Borden in Ontario, Canada; validate our parameter estimates against manually‐measured drawdown records in 14 other piezometers at Borden; and compare (a) our estimates of aquifer parameters with those obtained on the basis of all these records by Moench [2008], (b) on the basis of 11 transducer‐measured drawdown records by Endres et al. [2007], (c) our estimates of van Genuchten–Mualem parameters with those obtained on the basis of laboratory drainage data from the site by Akindunni and Gillham [1992], and (d) our corresponding prediction of how effective saturation varies with elevation above the initial water table under static conditions with a profile based on water contents measured in a neutron access tube at a radial distance of about 5 m from the center of the pumping well. We also use our solution to analyze 11 transducer‐measured drawdown records from a 7 day pumping test conducted by University of Waterloo researchers at the Canadian Forces Base Borden in Ontario, Canada. We validate our parameter estimates against manually measured drawdown records in 14 other piezometers at Borden. We compare our estimates of aquifer parameters with those obtained on the basis of all these records by Moench (2008) and on the basis of 11 transducer‐measured drawdown records by Endres et al. (2007), and we compare our estimates of van Genuchten–Mualem parameters with those obtained on the basis of laboratory drainage data from the site by Akindunni and Gillham (1992); finally, we compare our corresponding prediction of how effective saturation varies with elevation above the initial water table under static conditions with a profile based on water contents measured in a neutron access tube at a radial distance of about 5 m from the center of the pumping well.

A revisit of drawdown behavior during pumping in unconfined aquifers

In this study, the S‐shaped log‐log drawdown‐time curve typical of pumping tests in unconfined aquifers is reinvestigated via numerical experiments. Like previous investigations, this study attributes the departure of the S shape from the drawdown‐time behavior of the confined aquifer to the presence of an “additional” source of water. Unlike previous studies, this source of water is reinvestigated by examining the temporal and spatial evolution of the rate of change in storage in an unconfined aquifer during pumping. This evolution is then related to the transition of water release mechanisms from the expansion of water and compaction of the porous medium to the drainage of water from the unsaturated zone above the initial water table and initially saturated pores as the water table falls during the pumping of the aquifer. Afterward, the 1‐D vertical drainage process in a soil column is simulated. Results of the simulation show that the transition of the water release mechanisms in the 1‐D vertical flow without an initial unsaturated zone can also yield the S‐shaped drawdown‐time curve as in an unconfined aquifer. We therefore conclude that the transition of the water release mechanisms and vertical flow in the aquifer are the cause of the S‐shaped drawdown‐time curve observed during pumping in an unconfined aquifer. We also find that the moisture retention characteristics of the aquifer material have greater impact than its relative permeability characteristics on the drawdown‐time curve. Furthermore, influences of the spatial variability of saturated hydraulic conductivity, specific storage, and saturated moisture content on the drawdown curve in the saturated zone are found to be more significant than those of other unsaturated properties. Finally, a cross‐correlation analysis reveals that the drawdown at a location in a heterogeneous unconfined aquifer is mainly affected by local heterogeneity near the pumping and observation wells. Applications of a model assuming homogeneity to the estimation of aquifer parameters as such may require a large number of observation wells to obtain representative parameter values. In conclusion, we advocate that the governing equation for variably saturated flow through heterogeneous media is a more appropriate and realistic model that explains the S‐shaped drawdown‐time curves observed in the field.

The Analytical Solution of the Boussinesq Equation for Flow Induced by a Step Change of the Water Table Elevation Revisited

The classical problem of flow induced by a sudden change of the piezometic head in a semi-infinite aquifer is re-examined. A new analytical solution is derived, by combining an expression describing the water table elevation upstream, obtained by the Adomian’s decomposition approach, to an existing polynomial expression (Tolikas et al. in Water Resour Res 20:24–28, 1984), adequate for the downstream region; the parameters of both approximations are computed by matching the two solutions at the inflection point of the water table. Although several analytical solutions are available in the literature, we demonstrate that the expression we have developed in this issue is the most accurate for strong or moderate non-linear flows, where the degree of non-linearity is defined as the ratio of the piezometric head elevation at the origin to the initial water table elevation. For this type of flows the perturbation-series solution of Polubarinova-Kochina, characterized by previous studies as the best available analytical solution provides physically unacceptable results, while the analytical solution of Lockington (J Irrig Drain Eng 123:24–27, 1997), used to check the accuracy of numerical schemes, underestimates the penetration distance of the recharging front.

Headcut erosive regimes influenced by groundwater on disturbed agricultural soils

A series of simulated rainfall experiments, testing several soils and slope gradients in a 10 m × 0.8 m laboratory flume, displayed close correlations between initial development of a water table at a 10 cm depth and highly erosive headcut formation. On some soils and gradients, highly erosive headcuts formed consistently and predictably within minutes or seconds of initial water table rise. However, headcuts alone were not good indicators of increased erosion. In most experiments some headcuts formed early, often when surface hydraulic parameter values reached established rill initiation thresholds, but resulted in little or no erosion increase. Later, at initial water table rise, other headcuts formed coincident with major erosion increase, often with surface hydraulic values then less than rill initiation thresholds. On the four soils tested, highly erosive headcuts never formed without groundwater development, except on steep 9° slopes. Common visual indicators such as headcut morphology and headcut advance rates were not effective means of determining either erosion or the existence of groundwater. Only local monitoring of subsurface moisture conditions with micro-standpipes and TDR aided in determining headcut processes and erosive regimes. Groundwater-influenced headcut formation was likely caused by increased soil pore-water pressures and decreased soil shear strengths in surface rainflow, not by sapping or seepage from the soil matrix. Highly erosive headcuts can thus form under common agricultural conditions where reductions in permeability, such as plow pans, exist near the surface – without the need for saturated soils. Headcut erosive regimes were also significantly influenced by soil type and slope gradient, with the greatest effects of groundwater on moderate slopes and fairly permeable soils.

Laboratory studies of the adsorption of two pesticides (diuron and tebuconazole) using a batch design and an experimental flume: influence of contact conditions

The adsorption of two commonly used pesticides, diuron and tebuconazole, on an organic substrate (hemp), which was chosen as an analogue for natural substrates often found in agricultural ditches, has been studied using three different contact schemes: (1) contact in a beaker using a modified batch method; (2) contact in an experimental flume, with a dry start substrate; and (3) same as (2) but with the substrate initially saturated with water. Changes in pesticides concentrations as a function of time as well as adsorption after seven hours have been determined for each case. The highest adsorption is observed for the experiments with a flume initially containing dry hemp. In that case, both the initial condition (hemp initially without water) and the contact conditions between the pesticide solution and the substrate appear to favour adsorption. The lowest adsorption is obtained for the flume containing hemp initially saturated with water. In that case, samples that were obtained at different depths inside the hemp using capillary tubes showed that only the superficial part of the hemp was in equilibrium with the surface water. The presence of an initial water table appears to be a limiting factor for pesticides penetration and further adsorption onto hemp. An intermediate adsorption is obtained for the modified batch method. These results highlight the pronounced influence on adsorption of both the initial hemp wetness conditions and the contact conditions between the pesticides solution and the substrate. This influence should be studied further to assess the potential of agricultural ditches to mitigate pesticides contamination in surface water.

A Second‐Order Accurate, Finite Volume–Based, Integrated Hydrologic Modeling (FIHM) Framework for Simulation of Surface and Subsurface Flow

Surface water, the vadose zone, and groundwater are linked components of a hydrologic continuum. In order to capture the interaction between different components of a hydrologic continuum and to use this understanding in water management situations, an accurate numerical model is needed. The quality of model results depends on accurate representation of the physical processes and the data describing the area of interest, as well as performance of the numerical formulation implemented. Here we present a physics‐based, distributed, fully coupled, second‐order accurate, upwind cell‐centered, constrained unstructured mesh based finite‐volume modeling framework (FIHM) that simultaneously solves two‐dimensional unsteady overland flow and three‐dimensional variably saturated subsurface flow in heterogeneous, anisotropic domains. A multidimensional linear reconstruction of the hydraulic gradients (surface and subsurface) is used to achieve second‐order accuracy. Accuracy and efficiency in raster data and vector‐boundary representations are facilitated through the use of constrained Delaunay meshes in domain discretization. The experiments presented here (i) explore the influence of initial moisture conditions, soil properties, anisotropy, and heterogeneity in determining the pressure head distributions in the vadose and saturated zones, (ii) show the existence of localized “flux rotation” phenomenon due to heterogeneous anisotropy, leading to the creation of convergence–divergence zones, (iii) show the influence of vertical drainage from unsaturated zone on the response of an unconfined aquifer to pumping, and (iv) show the effects of capillarity, saturation excess, infiltration excess, and initial water table location on determining the overland flow generation.

Airflow induced by pumping tests in unconfined aquifer with a low‐permeability cap

Most analytical and numerical models developed to analyze pumping test data focus on saturated flow below the water table. Traditionally the soil above the initial water table prior to pumping has been thought to have little influence on the test results and has usually been ignored. It is hypothesized that, if the unsaturated zone is capped by low‐permeability soil, airflow in the unsaturated zone may be developed during pumping and may have impact on the drawdown in the aquifer. A transient, three‐dimensional and variably saturated flow model is employed to simulate the pumping‐induced air and groundwater flows in both the saturated zone and unsaturated zone with a low‐permeability layer. The results demonstrate that negative pressure in the unsaturated zone can be generated by pumping. The negative pressure begins to appear as the drawdown rate increases to a maximum, approaches a peak before the drawdown rate becomes zero, and then gradually disappears. Drawdown obtained from the capped aquifer is much greater because the water in the pores in the unsaturated zone is sucked by the negative pressure and the gravity drainage from the pores is hampered. Consequently, the drawdown versus time curve does not conform to the traditional S‐shaped curve for an unconfined aquifer but is similar to that of a confined aquifer. If the airflow caused by the low‐permeability cap is ignored, the error in estimated drawdown could be over 80% for the specific parameters and aquifer configuration used in the study. The possible errors in parameter estimation when airflow is ignored are explored. Overall, the hydraulic conductivity of the aquifer can be overestimated and the specific yield of the aquifer underestimated if airflow is ignored. The estimation error for specific yield tends to be greater than that in hydraulic conductivity.

New approximate solutions of horizontal confined–unconfined flow

Summary New approximate solutions of horizontal confined–unconfined flow are developed to extend the solution of Li et al. [Li, L., Lockington, D.A., Barry, D.A., Parlange, J.-Y., Perrochet, P., 2003. Confined–unconfined flow in a horizontal confined aquifer. Journal of Hydrology 271, pp. 150–155] to more general hydraulic conditions. Both initial water table and storativity of the confined zone are considered. The Boltzmann transformation is applied to simplify the mathematical model with dimensionless variables and lumped parameters. A closed-form analytical solution is obtained for flow in the confined zone. A high-precision numerical solution with the Runge–Kutta method and an iterative process is proposed for nonlinear flow in the unconfined zone. For an initially dry aquifer, an approximate analytical solution is derived. In comparison with the initially dry condition, the initial water table condition will lead to a faster moving speed of the confined–unconfined conversion interface. The initial water table condition is beyond the scope of Li et al.’s (2003) method. The groundwater level could be overestimated by neglecting storativity of the confined zone. By introducing an equivalent storage coefficient, the developed new solutions can also be applied to analyze the unconfined flow in a double-layer aquifer system. In the case study on a confined–unconfined groundwater flow induced by rise of the water table in a reservoir of a hydropower station in China, the calculated distribution of groundwater level with the new solutions generally agrees with the observations at the field site.

Impact of manipulated drought and heavy rainfall events on peat mineralization processes and source‐sink functions of an acidic fen

Climate change models predict changes in precipitation patterns over the next several decades for northern temperate regions. Resulting fluctuations of the water level may drastically affect the source‐sink functions of peatlands. Here, we manipulated the water table level in an acidic, minerotrophic fen using drying and rewetting experiments to simulate summer drought and heavy rainfalls to estimate changes in peat decomposition and source‐sink functions. We found that carbon dioxide (CO2) formation rates and exoenzymatic activities increased in the most active surface layer during the initial water table drawdown; however, extreme drying did not further increase these activities. Activity stimulated in deeper oxygenated peat layers did not substantially contribute to CO2 emissions. Additionally, no phenol oxidase activity was determined. Rewetting of peat after drying did not lead to a CO2 flush like in mineral soils. Water table manipulations yielded a higher availability of nitrate, ferric iron, and sulfate and prolonged the onset of methane formation. Sulfate was exported to a nearby stream. We concluded that the increasing frequency of extreme weather conditions like summer droughts and heavy rainfalls might not affect carbon storage but instead strengthen the sink function for nitrate and ferrous iron and the source function for sulfate in peatlands.

Numerical Simulation of Groundwater Flow with Gradually Increasing Heterogeneity due to Clogging

Well injection replenishes depleting water levels in a well field. Observation well water levels some distance away from the injection well are the indicators of the success of a well injection program. Simulation of the observation well response, located a few tens of meters from the injection well, is likely to be affected by the effects of nonhomogeneous medium, inclined initial water table, and aquifer clogging. Existing algorithms, such as the U.S. Geological Survey groundwater flow software MODFLOW, are capable of handling the first two conditions, whereas time-dependent clogging effects are yet to be introduced in the groundwater flow models. Elsewhere, aquifer clogging is extensively researched in theory of filtration; scope for its application in a well field is a potential research problem. In the present paper, coupling of one such filtration theory to MODFLOW is introduced. Simulation of clogging effects during “Hansol” well recharge in the parts of western India is found to be encouraging.

Effect of water table drawdown on peatland dissolved organic carbon export and dynamics

AbstractPeatlands play an important role in the global carbon cycle, and loss of dissolved organic carbon (DOC) has been shown to be important for peatland carbon budgets. The objective of this study was to determine how net production and export of DOC from a northern peatland may be affected by disturbance such as drainage and climate change.The study was conducted at a poor fen containing several pool–ridge complexes: (1) control site–no water table manipulation; (2) experimental site–monitored for one season in a natural state and then subjected to a water table drawdown for 3 years; (3) drained site–subjected to a water table drawdown 9 years prior to monitoring. The DOC concentration was measured in pore water along a microtopographic gradient at each site (hummock, lawn and hollow), in standing water in pools, and in discharge from the experimental and drained sites. The initial water table drawdown released ∼3 g of carbon per square metre in the form of DOC, providing a large pulse of DOC to downstream ecosystems. This value, however, represents only 1–9% of ecosystem respiration at this site. Seasonal losses of DOC following drainage were 8–11 g of carbon per square metre, representing ∼17% of the total carbon exchange at the experimental study site. Immediately following water table drawdown, DOC concentrations were elevated in pore water and open water pools. In subsequent seasons, DOC concentration in the pool declined, but remained higher than the control site even 11 years after water‐table drawdown. This suggests continued elevated net DOC production under lower water table conditions likely related to an increase in vegetation biomass and larger water table fluctuations at the experimental and drained sites. However, the increase in concentration was limited to initially wet microforms (lawns and hollows) reflecting differences in vegetation community changes, water table and soil subsidence along the microtopographic gradient. Copyright © 2008 John Wiley & Sons, Ltd and Her Majesty the Queen in right of Canada.

Dynamics of Soil Surface Bulk Density: Role of Water Table Elevation and Rainfall Duration

Measurements of soil bulk density profiles combined with thin‐section analysis have been suggested to assess the structural seedbed degradation caused by rainfall. The effects of water table elevation and rainfall duration on surface sealing and seedbed slumping were studied on a repacked silt loam soil. Two initial water table elevations (0.3 and 0.7 m below the soil surface) and three simulated rainfall durations (15, 30, and 40 min at 30.5 mm h−1 followed by 180 min at 7 mm h−1) were used. Seedbed bulk density profiles were generated using x‐radiography of resin‐impregnated soil slices. Macroporosity measurements using image analysis and thin‐section observations showed that infilling of eroded particles in interaggregate voids and compaction of the infilled particles were the main sealing processes. Below the seal, the seedbed exhibited coalescence and welding of aggregates into larger units, which affected mainly macroporosity. A model of sealing, exponential decrease in bulk density with depth, and slumping, linear increase in bulk density with depth, adequately reproduced the measured bulk density profiles (regression RMSE range 0.057–0.106 Mg m−3). The change in surface bulk density increased with rainfall duration, whereas this factor did not significantly affect slumping. The highest initial water table elevation led to the highest soil surface and internal seedbed bulk densities. It was suggested that high values of soil water content led to a decrease in aggregate cohesion. Moreover, the number of wetting and drying cycles and the water content during these cycles were shown to increase the magnitude of slumping.

Reply to comment on ‘C.‐P. Tung, N.‐M. Hong, C.‐H. Chen, and Y.‐C. Tan. 2004. Regional daily baseflow prediction. Hydrological Processes 18: 2147–2164’

AbstractThe recession analysis introduced by Brutsaert and Nieber (1977) has been applied in many studies. Tung et al. (2004) stated that using time‐averaged flows to estimate coefficients for recession equations may result in bias. The reasons include improper time interval to estimate averaged flows and the coefficient a in the recession analysis depends on the initial water table. The latter implies that the coefficient a may be different for different recession events. Rupp and Woods (2008) commented that the bias can be made negligible by using time intervals that are appropriate to the time rate of change in discharge. However, the bias of estimating coefficient a can still be expected if flows from different recession events are used on a log‐log plot. It desires attention even if appropriate time interval is considered. Copyright © 2008 John Wiley & Sons, Ltd.

Maximization of Water Storage in Backfilled and Lined Channels and Dimples Subject to Evaporation and Leakage

A channel or axisymmetric dimple filled with a coarse porous material and aimed at a temporary storage of infiltrated water as a perched water table aquifer is studied. The bottom shape is varied based on the criterion of maximal water storage after a certain period of drainage and evaporation. Leakage into the vadose zone through a thin liner occurs with a specific discharge proportional to the pressure drop across the liner. Evaporation through a horizontal shrinking water table is spatially uniform. An ordinary differential equation, which follows from the mass balance condition, is solved either explicitly or numerically. The class of triangular, polynomial, and conical sections is studied. The shape of maximal water retention is calculated for a given initial stored water volume or water table width, evaporation intensity, liner thickness and conductivity, vadose zone pressure beneath the liner, and selected time interval between two sequential infiltration events.

Factors Controlling Instability of Homogeneous Soil Slopes under Rainfall

Rainfall-induced slope failure is a common geotechnical problem in the tropics where residual soils are abundant. Although the significance of rainwater infiltration in causing landslides is widely recognized, there have been different conclusions as to the relative roles of antecedent rainfall to landslides. The relative importance of soil properties, rainfall intensity, initial water table location and slope geometry in inducing instability of a homogenous soil slope under different rainfall was investigated through a series of parametric studies. Soil properties and rainfall intensity were found to be the primary factors controlling the instability of slopes due to rainfall, while the initial water table location and slope geometry only played a secondary role. The results from the parametric studies also indicated that for a given rainfall duration, there was a threshold rainfall intensity which would produce the global minimum factor of safety. Attempts have also been made to relate the findings from this study to those observed in the field by other researchers. Results of this parametric study clearly indicated that the significance of antecedent rainfall depends on soil permeability.

Modeling regional initiation of rainfall-induced shallow landslides in the eastern Umbria Region of central Italy

We model the rainfall-induced initiation of shallow landslides over a broad region using a deterministic approach, the Transient Rainfall Infiltration and Grid-based Slope-stability (TRIGRS) model that couples an infinite-slope stability analysis with a one-dimensional analytical solution for transient pore pressure response to rainfall infiltration. This model permits the evaluation of regional shallow landslide susceptibility in a Geographic Information System framework, and we use it to analyze susceptibility to shallow landslides in an area in the eastern Umbria Region of central Italy. As shown on a landslide inventory map produced by the Italian National Research Council, the area has been affected in the past by shallow landslides, many of which have transformed into debris flows. Input data for the TRIGRS model include time-varying rainfall, topographic slope, colluvial thickness, initial water table depth, and material strength and hydraulic properties. Because of a paucity of input data, we focus on parametric analyses to calibrate and test the model and show the effect of variation in material properties and initial water table conditions on the distribution of simulated instability in the study area in response to realistic rainfall. Comparing the results with the shallow landslide inventory map, we find more than 80% agreement between predicted shallow landslide susceptibility and the inventory, despite the paucity of input data.