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There have been conflicting findings on hydrological dynamics in tropical montane cloud forests (TMCFs)—attributed to differences in climate, altitude, topography, and vegetation. We contribute another observation-based comparison between a TMCF (8.53 ha; 1906 m.a.s.l.) and a tropical lowland rainforest (TLRF) (5.33 ha; 484 m.a.s.l.) catchment in equatorial Sabah, Malaysian Borneo. In each catchment, a 90° v-notch weir was established at the stream's outlet and instrumented with a water-level datalogger that records data at 10-min intervals (converted to discharge). A nearby meteorological station records rainfall at the same 10-min intervals via a tipping bucket rain gauge connected to a datalogger. Over five years, 91 and 73 storm hydrographs from a TMCF and a TLRF, respectively, were extracted and compared. Various hydrograph metrices relating to discharge and flashiness were compared between the TMCF and TLRF while controlling for event rainfall, rainfall intensity, and antecedent moisture. Compared to the TLRF, storm-event runoff in the TMCF was up to 169% higher, reflecting the saturated conditions and tendency for direct runoff. Instantaneous peak discharge was also higher (up to 6.6x higher) in the TMCF. However, despite high moisture and steep topography, stream responsiveness towards rainfall input was lower in the TMCF, which we hypothesise was due to its wide and short catchment dimensions. Baseflow was significantly correlated with API20, API10, and API7. Overall, we found that the TMCF had higher runoff, but higher moisture condition alone may not be sufficient to govern flashiness. [ABSTRACT FROM AUTHOR]

Increases in the frequency of large to extreme rainfall events are widely predicted with global warming, but evidence from the humid tropics is hampered by a paucity of long-term data. This paper assesses changes in daily rainfall magnitude–frequency and their geomorphological consequences in the equatorial environment of northern Borneo using (1) rare daily rainfall data series for 1906–2012 assembled from archival and current sources and (2) geomorphological process data from the Danum Valley area of eastern Sabah. Rainstorm changes are explored using (1) analysis of frequencies of daily falls above threshold values, (2) extreme-value analysis comparing differences in locally weighted scatterplot smoothing (LOESS) best-fit curves for successive 20-year periods and (3) a novel approach constructing graphs of long-term change in daily falls of 1- to 10-year return period derived from overlapping 20-year LOESS curves. Substantial increases, some statistically significant, since 1980 (intensifying since 1999) are detected at most stations in (1) the frequency of daily falls ≥50, 100 and 200 mm and (2) the magnitudes of daily falls with return periods of 0.2–5.0 years. The scale and temporal patterns of historical change vary with (1) coastal aspect and (2) different parts of the return period spectrum. The period 1921–1940 exhibited higher rainstorm magnitude–frequency than 1952–1979 and demonstrates the dangers of simple deductions derived from post-1960 data comparisons. Geomorphological responses indicated by long-term monitoring at Danum (eastern Sabah), where there has been a marked upswing in heavy rainfalls since 1999, include a threefold increase in slopewash rates, more frequent high flows and higher sediment loads. The very high soil pipe sediment yields, rates of pipe roof collapse and stream channel extension recorded from 2002 to 2012 in pipe systems at Danum may be a consequence of this upswing. Increased landsliding is also a likely consequence, exacerbated in logged, agricultural and urban terrain. [ABSTRACT FROM AUTHOR]

Understanding the intensity and duration of tropical rain events is critical to modelling the rate and timing of wet- canopy evaporation, the suppression of transpiration, the generation of infiltration-excess overland flow and hence to erosion, and to river responsiveness. Despite this central role, few studies have addressed the characteristics of equatorial rainstorms. This study analyses rainfall data for a 5 km2 region largely comprising of the 4 km2 Sapat Kalisun Experimental Catchment in the interior of northeastern Borneo at sampling frequencies from 1 min-1 to 1 day-1. The work clearly shows that most rainfall within this inland, forested area is received during regular short-duration events (<15 mm) that have a relatively low intensity (i.e. less than two 0.2 mm rain-gauge tips in almost all 5 mm periods). The rainfall appears localized, with significant losses in intergauge correlations being observable in minutes in the case of the typical mid-afternoon, convective events. This suggests that a dense rain-gauge network, sampled at a high temporal frequency, is required for accurate distributed rainfall-runoff modelling of such small catchments. Observed rain-event intensity is much less than the measured infiltration capacities, and thus supports the tenet of the dominance of quick subsurface responses in controlling river behaviour in this small equatorial catchment. [ABSTRACT FROM AUTHOR]