The aim of the chapter is to show some practical and theoretical results related to the water mass balance in the unsaturated soil layer. This mass balance, representing the water evaporated from or infiltrated into the soil, is one of the main parts of the hydrological cycle and it is of great importance in different fields like hydrology, meteorology and agriculture.The water vapour flux in the lower atmosphere is usually estimated, on the basis of meteorological data, using the surface energy balance method (Bowen ratio) or measuring the turbulent transport (eddy correlation method). Here the water mass balance at the soil surface is obtained, under simple assumptions, by means of soil moisture measurements at different depths. This friendly algorithm can be easily used if moisture data are available. The aforementioned soil moisture data were obtained in the framework of the international Mesoscale Alpine Programme (MAP), a multi-year programme which included a Special Observing Period (SOP) spanning from the 7th of September to the 15th of November 1999. During the SOP the forecasters identified some Intensive Observing Periods (IOPs) during which extra measurements were performed in some previously defined Alpine test areas. The quoted soil moisture data were measured by means of the Time Domain Reflectometry (TDR) technique in the alpine valley of the Toce River (Lago Maggiore MAP target area). This area, and in particular the Ticino-Toce watershed, presents one of the local precipitation maxima of the southern slope of the Alps. The TDR data were collected six times a day from April to November 1999 in the upper soil layer from the surface to a maximum depth of 70 cm.This kind of soil moisture observations, not so frequently available, results very useful both during dry periods and periods with strong precipitation. Indeed, the water mass balance obtained by the TDR data allows estimating the water loss or gain of a column of soil of unitary cross section. During a drying period the trend of the cumulative water loss indicates when the evaporation is atmosphere limited (first stage) and when it is soil limited (second stage). During a wetting period the difference between the rain gauge data and the water gain of the column of soil indicates a possible runoff. In the second part of the chapter the Richards equation is taken into account. This non linear equation is analytically solved, for the drying case, neglecting the gravity term and using the well-known Boltzmann transformation. This transformation, assuming a uniform initial condition and a boundary condition with a time discontinuity at the surface, allows treating the problem with only one variable. It is important to stress that the differential equation will give the soil water content evolution if the hydraulic diffusivity function is known; vice versa it will give the diffusivity if the soil water content is known. This procedure is an important tool because it can be used to choose the theoretical solution that fits the experimental data features. Some experimental soil moisture vertical profiles, for example, clearly show an inflection point which may not be modelled, for the drying process, if the hydraulic diffusivity is a monotonic increasing function of the volumetric water content. Finally, some situations of interest are analytically modelled using the linearized Richards equation. With this approximation arbitrary initial and boundary conditions can be assumed obtaining valid solutions representing the experimental data both during infiltration and evaporation periods.
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|Data di pubblicazione:||2005|
|Titolo:||WATER BALANCE IN SURFACE SOIL: ANALYTICAL SOLUTIONS OF FLOW EQUATIONS AND MEASUREMENTS IN THE ALPINE TOCE VALLEY|
|Autori:||M. MENZIANI; S. PUGNAGHI; S. VINCENZI; R. SANTANGELO|
|Titolo del libro:||Climate and Hydrology in Mountain Areas|
|Appare nelle tipologie:||Capitolo/Saggio|
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