Carla Saltalippi

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.

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.

Flood forecasting and infiltration modeling/Prévision de crue et modélisation de l’infiltration

Abstract Abstract The role of accuracy in the representation of infiltration on the effectiveness of real-time flood forecasting models was investigated. A simple semi-distributed model of conceptual type with adaptive estimate of hydraulic characteristics included in the infiltration component was selected. Infiltration was described by a very accurate approach recently formulated for complex rainfall patterns, or alternatively through a simpler formulation known as an extension of the classical time compression approximation. The results indicated that, for situations involving a significant rainfall variability in space, the inaccuracy in the representation of infiltration cannot be corrected by the adaptive component of the rainfall–runoff model. A preliminary analysis of the role of an approximation of saturated hydraulic conductivity to be used in each homogeneous area of the semi-distributed model used both in non-adaptive version and in real-time is also presented.

An experimental hydrometeorological investigation to address infiltration-redistribution modelling

Experimental evidence of the time evolution of the soil moisture vertical profile in homogeneous soils under natural conditions is given in order to address the corresponding simulation modelling. Each experiment was conducted over two adjacent plots with soils of silty loam and loam type. The water content profiles θ(z,t) were continuously monitored using the time domain reflectometry method. For each profile four buriable three-rod waveguides were inserted horizontally at different depth (5, 15, 25 and 35 cm). In addition, we used sensors of air temperature, relative humidity, wind speed, solar radiation and rain as supports for the application of a specific simulation model. The results indicate that, under natural conditions, very different trends of the θ(z,t) function can be observed in a given fine-textured bare soil, where because of the formation of a crust layer an adjustment of the commonly used simulation modelling is required. In addition, because of the considerable variations of the moisture content vertical profiles measured in different soils types a generalization of the existing models is needed.

A Preliminary Analysis of Field-Scale Infiltration into Layered Soils

A preliminary analysis directed towards developing a model for the expected arealaverage infiltration into a layered soil profile is presented to represent the random spatial variability of the saturated hydraulic conductivity. The basic framework investigated for the model is the one proposed earlier for vertically homogeneous soils, that should be combined with the model for local infiltration into a two layered soil profile. In this context the possibility of reducing the complexity of a local modelling, which relies on the numerical solution of a system of two ordinary differential equations, is analysed by developing parameterized relations for the time to ponding at the surface and the suction head at the bottom of the upper layer.