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Closing the Gaps in Our Knowledge of the Hydrological
Cycle over Land: Conceptual Problems
William A. Lahoz
•
Gabri
¨
lle J. M. De Lannoy
Received: 9 October 2012 / Accepted: 25 January 2013 / Published online: 15 March 2013
The Author(s) 2013. This article is published with open access at Springerlink.com
Abstract This paper reviews the conceptual problems limiting our current knowledge of
the hydrological cycle over land. We start from the premise that to understand the
hydrological cycle we need to make observations and develop dynamic models that
encapsulate our understanding. Yet, neither the observations nor the models could give a
complete picture of the hydrological cycle. Data assimilation combines observational and
model information and adds value to both the model and the observations, yielding
increasingly consistent and complete estimates of hydrological components. In this review
paper we provide a historical perspective of conceptual problems and discuss state-of-the-
art hydrological observing, modelling and data assimilation systems.
Keywords Hydrological cycle
Earth observation
Land surface models
Data assimilation
1 Introduction
The water stored on land is a key variable controlling numerous processes and feedback
loops within the climate system (see, e.g., Dirmeyer
2000
; Koster et al.
2004a
,
b
; Sene-
viratne et al.
2010
). It constrains plant transpiration and photosynthesis and thus is of major
relevance for the Earth's water and energy cycles and impacts the exchanges of trace gases
on land, including carbon dioxide. Figure
1
, from IPCC (
2007
), provides an overview of
the main terrestrial components and exchanges within the climate system. This shows the
complexity of land processe
s and feedbacks, to a large extent owing to the high spatial
