Modelling effects of changing rainstorm size-frequency on annual interception and catchment water budgets in Sabah, Malaysian Borneo

Evaluating likely hydrological consequences of predicted and current actual climatic change is a complex challenge, not only because of uncertainties about societal, land-use and vegetational responses and feedbacks, but also because lack of information on some climatic variables that influence hydrological processes.  With a focus on the humid tropics, this paper addresses the influence of changes in rainstorm size-frequency on the interception component of evapotranspiration - and how this can in turn influences the nature and magnitude of impact of rainfall change on catchment water budgets.  This is critical for two reasons.  First, both climatic modelling predictions and current trends for annual rainfall in the humid tropics vary greatly between regions from significant increases to major declines.  Second, because of the ability of a warmer atmosphere to hold more water vapour, IPCC confidently predicts that extreme rainstorms worldwide will increase regardless of annual rainfall trends and it is also arguable that rainstorms in general will increase in intensity and storm total. Thus changes in rainstorm size-frequency distribution are to be expected.   

This paper focuses on the primary and disturbed equatorial rainforest environment of Sabah, Malaysian Borneo.  The paper (1) uses long-term daily rainfall series at four stations in Sabah (from 1906 for Sandakan, Tawau and Kota Kinabalu; and from 1985 for Danum) to assess the magnitude of recent changes in rainstorm size-frequency distribution in Sabah and (2) presents and uses a simple Excel-based model to translate these data into estimated changes in interception (and also total evapotranspiration and river flow).  The underlying principle of the model is that Interception (as a percentage of storm rainfall) falls with increasing storm size, as interception storage capacity is filled.  Results of previous rainforest interception studies (including by one of the authors at Danum in Sabah) are used to calibrate the model, whereby percentage interception reduces in steps from 100 % for storms of < 1 mm and 80 % for storms of 1-2 mm, to 13.6 % for storms of 10-14.9 mm and the interception capacity of 2.2 mm (5.5 % or less) for storms of >40 mm.  It follows that annual interception, both in mm and as a percentage of annual rainfall, will be much greater if most rainstorms are small, but progressively lower as the percentage of rain falling in big storms increases.  Differences in rainstorm size-frequency distribution help to account for the big range (7-27 % of annual rainfall) in annual interception found between different rainforest locations.  The Pico presentation first presents the model and demonstrates the magnitude by which simulated annual interception values vary between individual years of contrasting annual rainfall and rainstorm size-frequency distribution at the four locations.  Then the temporal changes in daily rainfall size-frequency at the four stations are presented and compared and  (using model outputs) changes in seasonal and annual interception and other water budget variables are assessed.  Finally, problems with, possible improvements to, and the wider applicability of the approach adopted are discussed.