Corrado Corradini

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.

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.

Initial Soil Water Content as Input to Field-Scale Infiltration and Surface Runoff Models

Evidence is given of the role of initial soil moisture content, θi, in determining the surface runoff hydrograph at field scale, that is a crucial element when distributed models for the estimate of basin response to rainfall have to be formulated. This analysis relies upon simulations performed by a model that, because of the necessity of representing the infiltration of surface water running downslope into pervious saturated or unsaturated areas, uses a coupled solution of a semi-analytical/conceptual approach for local infiltration and a nonlinear kinematic wave equation for overland flow. The model was applied to actual spatial distributions of θi, earlier observed over different fields, as well as to a uniform value of θi assumed equal to the average value or to the value observed in a site characterized by temporal stability. Our results indicate that the surface runoff hydrograph at a slope outlet is characterized by a low sensitivity to the horizontal heterogeneity of θi, at least in the cases of practical hydrological interest. In fact, in these cases the correct hydrograph can be simulated with considerable accuracy replacing the actual distribution of θi by the corresponding average value. Moreover, the surface hydrograph is sufficiently well reproduced even though a single value of θi, observed at a site anyhow selected in the field of interest, is used. In particular, this extreme simplification leads to errors in magnitude on peak runoff and total volume of surface water with values typically within 10% and 15%, respectively.

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.

Spatial structure of rainfall in mid-latitude cold front-systems

Abstract Experimental evidence and physical interpretation of rainfall spatial distribution associated with cold front systems in an inland region (2282 km2) of the Mediterranean area are provided. By using a representation on the meso-local scale, postfrontal and prefrontal storm ensembles consisting of thirty-two and seven components, respectively, were considered. For postfrontal storms the distribution of storm total depth was found to be mainly determined by small mesoscale precipitation areas of random nature with orographic effects that in the average were represented by a ratio of 1.5 between rainfall depth over the hills and in a lowland area just upwind of them. However, despite the basic role of random effects it was possible to divide the study region into a restricted number of homogenous zones whose structure appears to be linked with orography. For prefrontal storms the orographic enhancement was more considerable with the above ratio expressed by a value of 2.5. The enhancement was limited by the absence of contributions to surface rainfall due to a seeder-feeder process. The occurrence of an effect of inhibition of forced uplift, which accompanied both the storm types and was caused by fairly modest orography, is also discussed. Such a mechanism frequently produced a weakening or dissipation of pre-existing precipitation and may take on an important role in problems of rainfall analysis over real basins.

In situ measurements of soil saturated hydraulic conductivity: Assessment of reliability through rainfall–runoff experiments

The saturated hydraulic conductivity, Ks, is a soil property that has a key role in the partitioning of rainfall into surface runoff and infiltration. The commonly used instruments and methods for in situ measurements of Ks have frequently provided conflicting results. Comparison of Ks estimates obtained by three classical devices—namely, the double ring infiltrometer (DRI), the Guelph version of the constant-head well permeameter (GUELPH-CHP) and the CSIRO version of the tension permeameter (CSIRO-TP) is presented. A distinguishing feature in this study is the use of steady deep flow rates, obtained from controlled rainfall–runoff experiments, as benchmark values of Ks at local and field-plot scales, thereby enabling an assessment of these methods in reliably reproducing repeatable values and in their capability of determining plot-scale variation of Ks. We find that the DRI grossly overestimates Ks, the GUELPH-CHP gives conflicting estimates of Ks with substantial overestimation in laboratory experiments and underestimation at the plot scale, whereas the CSIRO-TP yields average Ks values with significant errors of 24% in the plot scale experiment and 66% in laboratory experiments. Although the DRI would likely yield a better estimate of the nature of variability than the GUELPH-CHP and CSIRO-TP, a separate calibration may be warranted to correct for the overestimation of Ks values. The reasons for such discrepancies within and between the measurement methods are not yet fully understood and serve as motivation for future work to better characterize the uncertainty associated with individual measurements of Ks using these methods and the characterization of field scale variability from multiple local measurements.