Field measurements of topsoil moisture p

Field measurements of topsoil moisture profiles

by vertical TDR probes

Roberto Greco *, Andrea Guida

Dipartimento di Ingegneria Civile, CIRIAM – Centro Interdipartimentale di Ricerca in Ingegneria Ambientale,

Seconda Universita` di Napoli, Via Roma 29, 81031 Aversa (CE), Italy

Received 27 July 2006; received in revised form 27 July 2007; accepted 10 October 2007

KEYWORDS

Time domain

reflectometry;

Moisture profiles;

Inverse problems;

Infiltration;

Evaporation;

Field monitoring

Summary A recently developed inverse method for the estimation of water content pro￾files from single time domain reflectometry (TDR) waveforms in laboratory has been

adapted and applied to field measurements of topsoil moisture profiles in a pyroclastic

sandy loam. Three metallic probes of the lengths of 30 cm, 45 cm and 60 cm were verti￾cally installed in an experimental field for the measurement of vertical water content pro￾files. One 15 cm long probe was inserted vertically into soil surface and five 10.5 cm long

probes were buried horizontally at various depths for the measurement of local values of

mean water content by means of the classical TDR approach. The experimental campaign

lasted 28 days, during which daily rainfall heights and daily maximum and minimum tem￾peratures were measured at the experimental field. TDR waveforms acquisition was car￾ried out twice a day. The agreement between local volumetric water content

measurements and vertical profiles was in general satisfactory, although some of the ver￾tical profiles failed in detecting a layer with systematically smaller water content values

indicated by the horizontal probe buried at the depth of 30 cm below soil surface. Such

small water content values could be probably ascribed to the presence of a large amount

of pumice stones in the soil around that depth, affecting the water content measured by

TDR probes and thus increasing estimated moisture spatial variability.

ª 2007 Elsevier B.V. All rights reserved.

Introduction

Time domain reflectometry (TDR) has been widely used in

the last decades for monitoring topsoil water content. In￾deed, TDR provides easy and cheap water content estima￾tions with relatively small disturbance to the investigated

soil. TDR measurement of soil water content, based on

the strong correlation observed between relative dielectric

permittivity of wet soil and its volumetric water content h

(Campbell, 1990), consists of measuring travel time Tp of

an electromagnetic pulse along a metallic waveguide of

known length Lp inserted into the soil. The volume averaged

value of soil relative dielectric permittivity er, affecting the

0022-1694/$ - see front matter ª 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.jhydrol.2007.10.013

* Corresponding author.

E-mail address: [email protected] (R. Greco).

Journal of Hydrology (2008) 348, 442– 451

available at www.sc iencedirect.com

journal homepage: www.elsevier.com/locate/jhydrol