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Climatic records for Danum for 1985-1998, elsewhere in Sabah since 1879, and long monthly rainfall series from other rainforest locations are used to place the climate, and particularly the dry period climatology, of Danum into a world rainforest context. The magnitude frequency and seasonality of dry periods are shown to vary greatly within the world's rainforest zone. The climate of Danum, which is aseasonal but subject, as in 1997-1998, to occasional drought, is intermediate between less drought-prone north-western Borneo and the more drought-prone east coast. Changes through time in drought magnitude frequency in Sabah and rainforest locations elsewhere in South-East Asia and in the Neotropics are compared. The 1997-1998 ENSO-related drought event in Sabah is placed into a historical context. The effects of drought on tree growth and mortality in the tropics are assessed and a model relating intensity and frequency of drought disturbance to forest structure and composition is discussed.

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In an earlier special issue of this journal, Marsh & Greer summarized forest land use in Sabah at that time and gave an introduction to the Danum Valley Conservation Area. Since that assessment, during the period 1990–2010, the forests of Sabah and particularly those of the ca 10 000 km 2 concession managed on behalf of the State by Yayasan Sabah (the Sabah Foundation) have been subject to continual, industrial harvesting, including the premature re-logging of extensive tracts of previously only once-logged forest and large-scale conversion of natural forests to agricultural plantations. Over the same period, however, significant areas of previously unprotected pristine forest have been formally gazetted as conservation areas, while much of the forest to the north, the south and the east of the Danum Valley Conservation Area (the Ulu Segama and Malua Forest Reserves) has been given added protection and new forest restoration initiatives have been launched. This paper analyses these forest-management and land-use changes in Sabah during the period 1990–2010, with a focus on the Yayasan Sabah Forest Management Area. Important new conservation and forest restoration and rehabilitation initiatives within its borders are given particular emphasis.

This paper investigates water repellency and soil moisture under 4 different Eucalyptus globulus plantations in Portugal. On 8 occasions over a 16-month period, measurements were made at 3 depths (surface, 0.10 and 0.20 m) at 60 points on four 10 by 18 m grids. The main results are: (i) at all sites and depths, spatial frequency of repellency (defined as percentage of repellent grid points) followed a moisture-related seasonal cycle, its amplitude being greatest for the longest established site, where surface repellency was contiguous in dry late-summer conditions, but was entirely absent after wet winter conditions; (ii) at a few points at 2 sites, repellency persisted during winter; (iii) repellency severity was dichotomously distributed regardless of season (i.e. soils were generally either wettable or highly repellent); and (iv) at the longest established site, when soil moisture was <14% soils were repellent, and when soil moisture was >27% soils were wettable. This may either support the existence of a ‘transition zone’, or be an artefact of the different scales of repellency and soil moisture assessments. Reasons for the observed changes in repellency and their relationship with soil moisture and antecedent rainfall are explored and soil hydrological implications discussed.

Abstract:Soil water repellency is generally thought to enhance runoff responses, thus representing a potentially important factor in hydrological modelling. Attempts to quantify its impacts have, however, either focused on soil profiles or plot scales and/or have been unable unequivocally to isolate repellency effects from other hydrologically important parameters. This research gap is addressed here by comparing responses of the same soil or terrain at a range of scales in highly repellent and wettable states, thereby limiting the impact of other variables. Hydrological responses of forest soils in north-central Portugal are assessed using laboratory wetting and rainfall simulation experiments at the point (0Ð002 m 2 ) and microplot scales (0Ð12 m 2 ), and field runoff responses from plot (16 m 2 ) and catchment scales (0Ð33 km 2 ). At the point scale, water repellency reduced soil wetting by a factor of >700 and increased mean runoff for a microplot from 1Ð5 to 53% during the first 9 min of a simulated rainstorm. At the field-plot scale impacts are evident, but less dramatic, with 16 of 45 of storms following dry, repellent conditions producing >3% overland flow compared with only one of 60 storms following moist, wettable conditions. In contrast, at the catchment scale, storm runoff responses were lower following dry, repellent, conditions than with moist, wettable conditions, although the time-topeak was on average 62% shorter for the former compared with the latter conditions. The reduced correspondence between the scales is attributed mainly to increased capture of locally generated overland flow at the catchment scale by bypass routeways that are under-represented at point and plot scales.It is concluded that addressing water repellency in distributed hydrological modelling has considerable potential for improving predictions for affected regions, but that impacts may vary considerably from those found in the Portuguese case example. Key parameters for determining its impacts are its severity, spatial distribution, transient characteristics, and the character and spatial distribution of any bypass routeways and wettable subsoil.

Archival rainfall data are used to investigate changes in drought frequency and severity in Sabah and other parts of northern Borneo since the late nineteenth century. Two measures of drought severity are used: drought duration (given by the number of consecutive months with less than 100 mm rain); and drought intensity (indexed by the cumulative rainfall deficit below 100 mm per month of a drought sequence). Within northern Borneo dry periods are very short (>3 months) and infrequent in Sarawak, south-western Sabah, Brunei and central and western Kalimantan; droughts occur seasonally, but are comparatively short in north-western Sabah; droughts are less frequent) but more severe in eastern Sabah and parts of eastern Kalimantan. In coastal Sabah and Brunei, there has been a statistically significant increase in the frequency and severity of droughts since the late 1960s. At Sandakan, two drought-prone epochs in 1877–1915 and 1968–92 (each experiencing five droughts of at least 4 months duration) are identified, separated by a 52-year period that was nearly drought-free. At Sandakan also, the ecologically damaging 1982–3 drought was neither as long or severe as those of 1903 and 1915. Links with El Niño-Southern Oscillation events are found to be not as strong as previous studies have suggested. Possible implications of the spatial and temporal patterns in drought magnitude-frequency for differences in tropical rain forest character within the region are discussed.

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