F. Melone

Assimilation of Surface- and Root-Zone ASCAT Soil Moisture Products Into Rainfall–Runoff Modeling

Nowadays, the availability of soil moisture estimates from satellite sensors offers a great chance to improve real-time flood forecasting through data assimilation. In this paper, two real data and two synthetic experiments have been carried out to assess the effects of assimilating soil moisture estimates into a two-layer rainfall-runoff model. By using the ensemble Kalman filter, both the surface- and root-zone soil moisture (RZSM) products derived by the Advanced SCATterometer (ASCAT) have been assimilated and the model performance on flood estimation is analyzed. RZSM estimates are obtained through the application of an exponential filter. Hourly rainfall-runoff observations for the period 1994-2010 collected in the Niccone catchment (137 km2), Central Italy, are employed as case study. The ASCAT soil moisture products are found to be in good agreement with the modeled soil moisture data for both the surface layer (correlation coefficient (R) of 0.78) and the root zone (R = 0.94). In the real data experiment, the assimilation of the RZSM product has a significant impact on runoff simulation that provides a clear improvement in the discharge modeling performance. On the other hand, the assimilation of the surface soil moisture product has a small effect. The same findings are also confirmed by the synthetic twin experiments. Even though the obtained results are model dependent and site specific, the possibility to efficiently employ coarse resolution satellite soil moisture products for improving flood prediction is proven, mainly if RZSM data are assimilated into the hydrological model.

Modeling infiltration for multistorm runoff events

We present a relatively simple analytical/conceptual model for rainfall infiltration during complex storms. It is an approximate but physically based model which can treat intervals of either no rain, low rain, or evaporation. The infiltration model is based on the very general three-parameter analytic model of Parlange et al. (1982), extended to treat soils with very high initial water content. The redistribution model is based on profile extension with shape similarity. A wide range of soil types can be simulated. The model is tested by comparison with numerical solutions of Richardss equation carried out for a variety of events upon four selected soils. The model simulates the solution to Richardss equation quite accurately, provided basic soil retention relations are parametrically represented. It simulates redistribution particularly well for redistribution intervals up to 20 hours. The model usefulness in comparison with the common and simple approach which disregards soil water redistribution is also shown.

On the interaction between infiltration and Hortonian runoff

Abstract A model which couples the kinematic wave approximation for Hortonian overland flow and the conceptual approach developed by Corradini et al. (1997) [J. Hydrol., 192, 104–124] for local infiltration was used to investigate the effects of random spatial variability of saturated hydraulic conductivity, K s , on the outflow hydrograph at hillslope scale. The model incorporates a representation of infiltration of overland flow running over pervious downstream areas (“run-on” process). Single rainfall pulses and complex storms over two soils, representative of a silty loam and a sandy loam soil, were considered. Our results suggest that for realistic values of the coefficient of variation of K s the run-on process cannot be disregarded, because it produces a significant decrease of overland flow during both the rising and the recession limb of the hydrograph. Furthermore, the role of the level of spatial correlation of K s was found to be typically minor and the run-on process concurred to this result. The possibility of simplifying the stochastic problem by a deterministic approach based on the use of a uniform lumped value of K s was also examined and rainfall patterns adequate for this simplification were deduced in terms of scaled storm intensity and storm duration.

A unified model for infiltration and redistribution during complex rainfall patterns

Abstract A relatively simple conceptual model for infiltration during complex rainfall sequences is presented. It is a reformulation of an analytically derived model developed earlier by Smith et al. (1993) [Water Resour. Res., 29(1): 133–144] and Corradini et al. (1994) [Water Resour. Res., 30(10): 2777–2784] in a more homogeneous version suitable for hydrologic applications. The model relies on a single ordinary differential equation through which successive cycles of infiltration-redistribution can be described. The wetting profile, σ(z), is described by a single similarity curve which is stretched in both z and σ dimensions in accordance with the curve area, I, representing infiltrated water. The actual profile of soil water content after rewetting may be compound, composed two parts, each part represented by a similarity profile shape which evolves in time. The model is calibrated using a silty loam soil and is then tested on a variety of soils, from fine-textured to sandy loam soil types, by comparison with a numerical solution of the Richards equation. Like the earlier formulation, the model simulates infiltration rate particularly well, and the greater simplicity of this model sacrifices very little accuracy of the earlier, more complex model.

Modeling local infiltration for a two-layered soil under complex rainfall patterns

Abstract A general model for local infiltration–redistribution–reinfiltration in a two-layered soil profile is proposed as an extension and generalization of a formulation we developed earlier for infiltration and redistribution in crusted soils. It is of analytical/conceptual type and involves a Runge–Kutta–Verner numerical solution of a system of two ordinary differential equations. Given the basic hydraulic properties of soils, there are practically no parameters to estimate by calibration. The wetting profile during infiltration is represented by a single similarity curve, while in the reinfiltration stage, which occurs for rewetting after a period of redistribution, the profile of soil water content in each horizontal layer may be composed of two similarity profiles which evolve in time. The model is tested by comparison with numerical solution of Richards’ equation, carried out for a variety of synthetic experiments upon the combination of three soil types of contrasting hydraulic characteristics. It simulates particularly well the entire cycle infiltration–redistribution–reinfiltration. In all cases, where either layer may be less permeable, the water content at the surface and the interface as well as the infiltration rates are very accurately estimated. The usefulness of compound profiles of water content in the reinfiltration stage is also shown by comparison with the simple approach, which relies on a single similarity curve.

Improving Landslide Forecasting Using ASCAT-Derived Soil Moisture Data: A Case Study of the Torgiovannetto Landslide in Central Italy

Predicting the spatial and temporal occurrence of rainfall triggered landslides represents an important scientific and operational issue due to the high threat that they pose to human life and property. This study investigates the relationship between rainfall, soil moisture conditions and landslide movement by using recorded movements of a rock slope located in central Italy, the Torgiovannetto landslide. This landslide is a very large rock slide, threatening county and state roads. Data acquired by a network of extensometers and a meteorological station clearly indicate that the movements of the unstable wedge, first detected in 2003, are still proceeding and the alternate phases of quiescence and reactivation are associated with rainfall patterns. By using a multiple linear regression approach, the opening of the tension cracks (as recorded by the extensometers) as a function of rainfall and soil moisture conditions prior the occurrence of rainfall, are predicted for the period 2007–2009. Specifically, soil moisture indicators are obtained through the Soil Water Index, SWI, a product derived by the Advanced SCATterometer (ASCAT) on board the MetOp (Meteorological Operational) satellite and by an Antecedent Precipitation Index, API. Results indicate that the regression performance (in terms of correlation coefficient, r) significantly enhances if an indicator of the soil moisture conditions is included. Specifically, r is equal to 0.40 when only rainfall is used as a predictor variable and increases to r = 0.68 and r = 0.85 if the API and the SWI are used respectively. Therefore, the coarse spatial resolution (25 km) of satellite data notwithstanding, the ASCAT SWI is found to be very useful for the prediction of landslide movements on a local scale. These findings, although valid for a specific area, present new opportunities for the effective use of satellite-derived soil moisture estimates to improve landslide forecasting.

Modeling infiltration during complex rainfall sequences

An extension of the conceptual model earlier developed by Smith et al. (1993) is presented. Their basic model considered the problem of point infiltration during a storm consisting of two parts separated by a rainfall hiatus, with surface saturation and runoff occurring in each part. The model is here extended toward further generality, including the representation of a sequence of infiltration-redistribution cycles with situations not leading to soil surface saturation, and rainfall periods of intensity less than the soil infiltration capacity. The model employs at most a two-part profile for simulating the actual one. When the surface flux is not at capacity, it uses a slightly modified version of the Parlange et al. (1985) model for description of increases in the surface water content and the Smith et al. (1993) redistribution equation for decreases. Criteria for the development of compound profiles and for their reduction to single profiles are also incorporated. The extended model is tested by comparison with numerical solutions of Richardss equation, carried out for a variety of experiments upon two contrasting soils. The model applications yield very accurate results and support its use as part of a watershed hydrologic model.

A conceptual model for infiltration and redistribution in crusted soils

Treating the crust as a single layer, a simplified but accurate infiltration and redistribution model for a crust-topped soil profile is developed. The method is an extension of an earlier infiltration and redistribution model for homogeneous soil profiles. The crust layer is conceptually subdivided into regions associated with surface water content and interface water content, respectively. The extension of the wetting front into the subsoil is described by the homogeneous soil method described earlier. Two ordinary differential equations are written which match pressure heads and fluxes at the soil interface and are solved by a Runge-Kutta technique. The model is tested against the results of the Richards equation with variations in the key variables of capillary length scale and saturated hydraulic conductivity for both soil and crust and for cases where ponding occurs both in the crust region and after wetting has entered the subsoil region. In all cases the profile water contents and infiltration rates are very reasonably simulated.

Case Study: Improving Real-Time Stage Forecasting Muskingum Model by Incorporating the Rating Curve Model

Analysis of forecasts obtained by a forecasting model called STAFOM, a Muskingum-type model for real-time application, shows that the model provides accurate forecast stage estimates for most of the selected case studies and flood events in the Upper-Middle Tiber River basin in central Italy. However, three main issues affected STAFOM: (1) its kinematic nature, (2) the lateral inflows representation, and (3) the occurrence of sudden fluctuations in water levels observed at the ends of the equipped river reach. Therefore, this simple stage forecasting model is hereby improved by incorporating a methodology relating local stage and remote discharge along river channels. This latter procedure, based on the rating curve model (RCM), is capable of reconstructing a discharge hydrograph at a river site where only the stage is monitored, while the discharge is recorded at another section located far away and for which a significant lateral inflow contribution is expected. Application of the new model, named STAFOM-RCM, to several flood events that occurred along four equipped river reaches of the Upper-Middle Tiber River basin, shows that it improves the stage forecast accuracy both in peak and stage hydrograph primarily for long river reaches, thus allowing consideration of a longer forecast lead time; and hence, avoiding the use of the old two-connecting river branch scheme that amplified the fluctuations in observed water levels. DOI: 10.1061/(ASCE)HE.1943-5584.0000345.

Simulation of the direct runoff hydrograph at basin outlet

A simple model describing the transformation of effective rainfall to direct runoff through the overland flow mechanism is presented. The model is based on the classical representation of a watershed by a combination of planes and channels. The dynamics of overland flow in each plane is simulated by the non-linear kinematic wave, but the outflow from a given plane is concentrated in the middle of the corresponding drainage channel. The water routing in the channels is carried out by a piece-wise linearized formulation in space of the kinematic wave approximation. Using synthetic events on 10 watersheds, the model was tested by comparing it with results obtained by applying the non-linear kinematic wave to all the elements of the watershed. The model was found to be adequate, even in a form that simplifies the geometric features of the planes through an averaging procedure based on the Horton–Strahler ordering scheme of the watershed.