BNPP/ASB Functional Value of Biodiversity Project – Phase II 

2.2 Hydrological models

For Activity 2, different hydrological models are being run in parallel across a nested set of ((sub) sub) catchments. The project will explore the complementarity and consistency of hydrological models with different ranges of scale and differing emphases in the representation of physical processes. The models to be used in the BNPP project belong to different categories in terms of time course of changes in soil properties affecting infiltration. We will analyze the assumptions underlying relevant hydrological models used at different spatial and temporal scales.

Differences among the various hydrological models used in Activity 2, as well as the synoptic Water Balance Model (WBM) which also is used in Activity 1, are summarized in Table 2. This table also shows the classification of hydrological models to be used in the BNPP project, according to 5 levels of complexity.

While the results of these models are not conclusive by themselves, they are the best analyses possible at this time in terms of modeling expertise and data used. Moreover, these simulations are essential in controlling the (strong) effects of climate and terrain on specific hydrological functions and thereby in isolating the (weaker) effects of land cover, which is the focus of many interventions. In predicting the quantities and timing of riverflow, models:

1.   will normally include rainfall, energy balance (potential evapotranspiration), soil storage capacity and landscape (routing times in the stream network) properties,

2.   they normally also include properties of the land cover (derived from remote sensing or otherwise) with respect to rainfall interception and actual evapotranspiration as a function of soil water storage, and thus respond to changes in area fractions of different land cover types,

3.   some may include influences of land cover on the infiltration capacity, and require data on rainfall intensity to predict surface runoff on sloping lands, and thus include effects of land use change on the peakflow/baseflow ratio,

4.   some may explicitly include overland flows, the entrainment of soil particles into this flow, and the sedimentation of soil particles in ‘filter’ zones, thus becoming sensitive to the spatial organization of the landscape,

5.   a few will actually treat the change in soil properties affecting infiltration as a dynamic process (rather than instantaneous change), and thus become sensitive to the time course of land use change, rather than just the final outcome in  terms of area fractions of different land cover types.

All models will be used for a comparison of ‘natural vegetation’ (baseline) versus ‘current land use pattern’, with current climate. A specific effort will be made to derive location-specific scenarios of plausible land use change, which will be evaluated for its bearing on hydrological functions. Further scenario development is described under activity c.

Models, if correctly implemented, allow for an explicit representation of the consequences of a series of assumptions. No model is correct, no model is wrong – but the assumptions may or may not be sufficient and necessary to reconstruct the phenomena that we can observe. As different modelers may have slightly different interpretations of the same set of assumptions, or differ in the assumptions they make, it is generally relevant to compare between different model implementations, even if they refer to broadly the same set of hypotheses. In the context of Activity 2, we will explore a number of models that were initially developed for different sets of circumstances, temporal and spatial scales. Before we represent results of these models, we thus need to clarify the various assumptions, similarities and differences.

See Appendix 6 for four steps for meso (& micro) scale models to clarify land use change effects on watershed functions.

Table 2. Hydrological Models

Questions at the level of model structure and assumptions

The simplistic ‘upland’ – ‘lowland’ construction of watershed management issues tends to ignore the importance of the ‘transmission zone’, but events in the riverbed can have a major impact on the actual performance of rivers.

Interactions between ‘lowland’ and ‘upland’ are driven by current ‘perceptions’ of the ‘hydro logic’, not by hard facts and measurable results.  The ‘emotional basis’ level needs to be balanced by better understanding of patterns, processes, threats and opportunities for better governance.

Roughly, three intervention points can be identified in the chain of events by which ‘land use change’ (as the more sophisticated version of ‘deforestation’), leads to human exposure to risks:

·        effects of land use pattern on the way rainfall is converted to streamflow,

·        hydrological properties of the stream network that  relate inputs to the streams to water levels and qualities downstream (‘engineering solutions’ can modulate this step)

·        location of human stakeholders relative to the streambed (not living on the wrong place on the wrong time makes a big difference)

·        Where the ‘knee jerk’ reaction in many watershed function cases is to look for the forests and land use change as explanations, we need to find a balance between ‘land use based’, ‘engineering’ and ‘locational/zoning’ solutions to current problems.

Table 3. Measurability of land use impacts by basin size (Kiersch and Tognetti, 2002) x = Measurable impact, - = No measurable impact.

The lack of ‘hard’ data for most watershed functions beyond 10 km² may be largely due to inadequacies in study design, but may also reflect the importance of other ‘drivers’ – a prime candidate is the ‘spatial variability in rainfall’ mechanism that the GenRiver + SpatRain model illuminates (as discussed below).

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Last updated: 28 November, 2003     ©2003 ASB. All rights reserved.