Percolation losses in paddy fields with a dynamic soil structure: Model development and applications

Autor: Janssen, M.; Lennartz, B.; Wöhling, Th.
In:

Hydrological Processes

Bandangabe: 24
ISBN: 0885-6087
Seite: 813 - 824
Jahr: 2010

Einordung:
Institut: Professur Bodenphysik
Teilschwerpunkt: Erfassung und Auswertung umweltrelevanter Kenngrößen

Abstract:
The hydraulic characteristics of the plough pan of paddy fields provide continuous ponding conditions during the growing
season and control the water use efficiency in wet rice production. Its saturated hydraulic conductivity Ks, however, exhibits
a large spatiotemporal variability as a consequence of a highly dynamic soil structure involving temporary shrinkage cracks.
Water flow through the earthen bunds surrounding the fields further contributes to the uncertainty in water flux calculations. The
objective of this study was to develop a simple deterministic model with stochastic elements (‘PADDY-FLUX’) for depiction
of deep percolation, and to assess the effect of different water management scenarios on percolation in two channel command
areas. Darcy’s law is used as the fundamental equation for water flow calculations with the ponding water depth h as a timedependent
variable. Flux uncertainty is estimated by a Monte-Carlo-type implementation. Ks is treated as a random variable of
a bimodal probability density function (PDF), which is the weighted sum of two Gaussian PDFs (accounting for a matrix and a
preferential flow domain). The weighing factor ˛ is a function of h, reflecting an increasing risk for preferential flow situations
after desiccation and the development of shrinkage cracks. Under-bund percolation is calculated using transfer functions. The
results demonstrate that percolation losses increase in the following order: continuous soil saturation < continuous flooding
(CF) < mid-season drainage and intermittent irrigation (MD C II) < mid-season drainage and continuous flooding. The bunds
contribute up to 54 and 17% to total fluxes under CF and MD C II, respectively. Preferential water fluxes are responsible for
the major part of water losses as soon as desiccation causes the formation of shrinkage cracks. As a conclusion, continuous
soil saturation should be promoted as the least water-intensive irrigation regime, while intermittent irrigation is recommended
only in case that irreversible shrinkage cracks have already developed.

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