PUB in practice: case studies - Runoff Prediction in Ungauged Basins: Synthesis across Processes, Places and Scales - page 350

(MRC, 2003 ). The MRB also constitutes an extremely
diverse and complex ecosystem, supporting some of the
world

Method
The YHyM was developed at the University of Yamanashi
(Japan), particularly for hydrological simulations of large
river basins (Takeuchi et al., 1999 , 2008 ; Ishidaira et al.,
2000 ;Aoet al., 2003 ; Zhou et al., 2006 ; Hapuarachchi
et al., 2008 ). It is a comprehensive grid-based distributed
hydrological model (see Section 10.4.1 ) integrated with
modules for estimating potential evapotranspiration, snow
accumulation/melt, runoff generation, sediment transport,
water quality and water use/control (e.g., dam/reservoir
operations).
The core hydrological model of YHyM is an extension
of the TOPMODEL concept (Beven and Kirkby, 1979 )
referred to as BTOPMC (blockwise TOPMODEL with
Muskingum - Cunge method). This extension was made
by redefining the topographical index by using an effective
contributing area per unit grid cell area and introducing
concepts of mean groundwater travel distance and ground-
water dischargeability. This provides a link between hill-
slope hydrology and macro-hydrology (Takeuchi et al.,
1999 , 2008 ). The BTOPMC model has four parameters
to be identified at each grid (maximum saturation deficit in
root zone, groundwater dischargeability, river width and
Manning

s highest diversities of fish and snails, as well as a
number of critically endangered species such as the Irra-
waddy dolphin and the giant Mekong catfish ( Yoshimura
et al., 2009 ). Approximately 60 million inhabitants sustain
their livelihood directly from the Mekong River, but the
Mekong River also provides food staples, mainly rice, for
about 300 million people (MRC, 2010 ).

'

Description of the study area
The Mekong River ( Figure 11.83 ) is the world s twelfth

longest river and tenth largest in terms of annual flow. The
climate of MRB ranges from tropical in the LMB to cool
temperate in the upper Mekong (China) where some of the
peaks in the Tibetan Plateau are permanently snow-capped.
Annual average precipitation in the basin is

'

1405 mm

~

and average evapotranspiration is 825 mm. About 55%

of the water in the lower basin arises from the mountainous
regions along the eastern rim of the basin, with north-east
Thailand contributing only 10% (MRC, 2003 ). Precipita-
tion in the basin mainly occurs between May and October,
associated with the south-west monsoon, and frequent
flooding is observed in different parts of the MRB during
this period. The north-east monsoon is from November to
March and is associated with the cool and dry conditions of
the MRB.
Compounding this high seasonal to inter-annual hydro-
climatic variability, and the population increases already
mentioned, are the potential impacts of anthropogenic cli-
mate change. In the Tropical Asia region where the MRB
is located, the potential climate change impacts include
strengthening of monsoon circulation, increases in surface
temperature, increases in the magnitude and frequency of
extreme rainfall events, and sea-level rise (Cruz et al.,
2007 ). These projected changes could result

~

s roughness coefficient) and one in each block
(i.e., sub-basin), which is the groundwater discharge decay
factor where the groundwater aquifer is assumed shared.
Regardless of this extension and redefinition, the
BTOPMC model uses all the original TOPMODEL equa-
tions in their basic form.
In this study we applied YHyM to the MRB to investi-
gate current (1980

'

99) hydro-
logical conditions (refer to Hapuarachchi et al.( 2008 ),
Kiem et al.( 2008 ) and Takeuchi et al.( 2008 ) for a full
description of the YHyM parameters and data inputs used
in the MRB case study). Here we focus on the performance
of YHyM in the ungauged, or poorly gauged, parts of the
MRB and also on YHyM ' s ability to simulate variables
other than runoff (e.g., soil moisture, actual evaporation),
with particular emphasis on how parameters were esti-
mated in the ungauged parts of the MRB.

- 2000) and future (2080 -

in major

impacts on the MRB s ecosystems and biodiversity;

hydrology and water resources; agriculture, forestry, and
fisheries; mountains and coastal lands; and human settle-
ments and human health. In order to adapt to these pro-
jected changes it is necessary to understand historical and
existing conditions (i.e., baseline) and also to robustly
quantify the impacts of projected future climatic changes,
particularly in relation to the current and projected hydrol-
ogy, water resources and water environments (e.g., Kiem
and Verdon-Kidd, 2011 ). This is problematic in large,
mostly ungauged, basins like the MRB. This case study
demonstrates how the Yamanashi Hydrological Model
(YHyM) can be used as a tool to assess current and future
water resource availability in large, poorly gauged basins
like the MRB.

'

Parameter estimation
The entire MRB was divided into nine blocks ( Figure
11.83 ), taking into consideration the size, natural sub-
basins, and the Köppen climate classification. One of the
main advantages of YHyM is that most of its parameters are
physically based, thus the number of parameters that require
tuning is parsimonious and the regionalisation of param-
eters is possible (e.g., Ao et al., 2006 ) (see Sections 10.4.3
and 10.4.4 ). Basic catchment characteristics such as eleva-
tion, slope, flow direction and river network were extracted
from a digital elevation model. The channel width was
assumed to be proportional to upstream catchment area