Global-scale regionalization of hydrologic model parameters

Current state-of-the-art models typically applied at continental to global scales (hereafter called macroscale) tend to use a priori parameters, resulting in suboptimal streamflow (Q) simulation. For the first time, a scheme for regionalization of model parameters at the global scale was developed. We used data from a diverse set of 1787 small-to-medium sized catchments ( 10-10,000 km(2)) and the simple conceptual HBV model to set up and test the scheme. Each catchment was calibrated against observed daily Q, after which 674 catchments with high calibration and validation scores, and thus presumably good-quality observed Q and forcing data, were selected to serve as donor catchments. The calibrated parameter sets for the donors were subsequently transferred to 0.5 degrees grid cells with similar climatic and physiographic characteristics, resulting in parameter maps for HBV with global coverage. For each grid cell, we used the 10 most similar donor catchments, rather than the single most similar donor, and averaged the resulting simulated Q, which enhanced model performance. The 1113 catchments not used as donors were used to independently evaluate the scheme. The regionalized parameters outperformed spatially uniform (i.e., averaged calibrated) parameters for 79% of the evaluation catchments. Substantial improvements were evident for all major Koppen-Geiger climate types and even for evaluation catchments>5000 km distant from the donors. The median improvement was about half of the performance increase achieved through calibration. HBV with regionalized parameters outperformed nine state-of-the-art macroscale models, suggesting these might also benefit from the new regionalization scheme. The produced HBV parameter maps including ancillary data are available via

Effects of Reforestation of a Degraded Imperata Grassland on Dominant Flow Pathways and Streamflow Responses in Leyte, the Philippines

Reforestation of degraded grasslands can increase the soil hydraulic conductivity and number of preferential flow pathways. However, it is not clear to what extent these changes affect streamflow responses and whether this depends on the event size. We, therefore, studied the hydrological response of two small catchments near Tacloban, Leyte (the Philippines): a degraded Imperata grassland catchment and a catchment that was reforested 23 years prior to our study. Precipitation, stream stage, and electrical conductivity were measured continuously from June to November 2013. Samples were taken from streamflow, precipitation, groundwater, and soil water for geochemical and stable isotope analyses. Streamflow and electrical conductivity changed rapidly during almost every event in the grassland catchment, but in the reforested catchment, these responses were much smaller and only occurred during large events. Streamflow was a mixture of groundwater and precipitation for both catchments, but the maximum event water contributions to streamflow were much larger for the degraded grassland than for the reforested catchment. The differences in the event water contributions and timing of the streamflow responses were observed across all event sizes, including a large tropical storm. Together with the low saturated hydraulic conductivity in the degraded catchment, these results suggest that overland flow occurred more frequently and was much more widespread in the degraded grassland than in the reforested catchment. We, therefore, conclude that reforestation of a degraded grassland can change the dominant flow pathways and restore the hydrological functioning if the forest soil is allowed to develop over a sufficiently long period

Water budget and run-off response of a tropical multispecies “reforest” and effects of typhoon disturbance

To examine claims that reforesting degraded Imperata grassland in Leyte (Philippines) made streamflow perennial again, we studied the hydrological behaviour of a 23‐year‐old mixed‐species ‘reforest' between June 2013 and May 2014. Typhoon Haiyan, one of the strongest events ever, caused major damage to the site in November 2013. Average daily apparent water use (ET) was 5.0 mm d‐1 pre‐typhoon and 3.2 mm d‐1 after disturbance. Corresponding ratios of period‐total quickflow Qq to precipitation were 16% and 44%. Quickflow volume and peak discharge increased rapidly once a threshold value of ~250 mm for soil water storage in the top 60 cm was exceeded. Before disturbance, quickflow consisted predominantly of lateral subsurface flow due to high soil hydraulic conductivities down to 60 cm. After disturbance, shallow groundwater rose regularly to within 10 cm of the surface on foot‐slopes and saturation overland flow was observed during several large storms. Comparing estimated annual ET and Qq for undisturbed conditions for the reforest and a nearby degraded Imperata grassland micro‐catchment suggested that the extra infiltration following reforestation (~240 mm yr‐1) exceeded the extra ET by the reforest (100–185 mm yr‐1), implying a net positive trade‐off (55–140 mm yr‐1) and tentatively confirming local claims of improved dry‐season flow

Effects of reforestation of a degraded Imperata grassland on dominant flow pathways and streamflow responses in Leyte, the Philippines

Reforestation of degraded grasslands can increase the soil hydraulic conductivity and number of preferential flow pathways. However, it is not clear to what extent these changes affect streamflow responses and whether this depends on the event size. We, therefore, studied the hydrological response of two small catchments near Tacloban, Leyte (the Philippines): a degraded Imperata grassland catchment and a catchment that was reforested 23 years prior to our study. Precipitation, stream stage, and electrical conductivity were measured continuously from June to November 2013. Samples were taken from streamflow, precipitation, groundwater, and soil water for geochemical and stable isotope analyses. Streamflow and electrical conductivity changed rapidly during almost every event in the grassland catchment, but in the reforested catchment, these responses were much smaller and only occurred during large events. Streamflow was a mixture of groundwater and precipitation for both catchments, but the maximum event water contributions to streamflow were much larger for the degraded grassland than for the reforested catchment. The differences in the event water contributions and timing of the streamflow responses were observed across all event sizes, including a large tropical storm. Together with the low saturated hydraulic conductivity in the degraded catchment, these results suggest that overland flow occurred more frequently and was much more widespread in the degraded grassland than in the reforested catchment. We, therefore, conclude that reforestation of a degraded grassland can change the dominant flow pathways and restore the hydrological functioning if the forest soil is allowed to develop over a sufficiently long period

Runoff response and sediment yield of a landslide-affected fire-climax grassland micro-catchment (Leyte, the Philippines) before and after passage of typhoon Haiyan

Decades of logging and slash-and-burn agriculture have turned vast tracts of land in tropical South-east Asia into unproductive fire-climax grasslands whose hydrological functioning is poorly known. To help fill this knowledge gap, a 3.2 ha landslide-affected Imperata grassland micro-catchment with perennial flow on Leyte Island (Philippines) was instrumented and monitored for a year. The area was hit by typhoon Haiyan on 8 November 2013, one of the largest events on record. Landslide surfaces covered 3.4% of the catchment prior to typhoon Haiyan and contributed to ‘direct runoff’ (Qq). This basic ‘contributing area’ increased to 7.7% by activation of old landslides and formation of new ones during typhoon Haiyan. Median storm runoff coefficients (Qq/P) based on straight-line hydrograph separation were 9% and 23% before (48 events) and after the typhoon (43 events), respectively, but the ratios of period-total Qq and P were much larger (24% and 47%, respectively). Both storm runoff volumes and peak discharge increased rapidly once a mid-slope water storage threshold for the upper 60 cm of soil of 250mm was exceeded. Storm runoff contributions above those generated on landslides were most likely in the form of overland flow given the prevailing very low soil hydraulic conductivities. Post-typhoon water use of the heavily disturbed vegetation was reduced initially by nearly 70%, recovering to nearly 80% of the pre-typhoon value after∼3 months. The high annual sediment yield (∼27 t ha−1) was heavily dominated by post-Haiyan sediment transport (94%); bedload contributed ∼8% of the total sediment yield

Water budget and run-off response of a tropical multispecies “reforest” and effects of typhoon disturbance

To examine claims that reforesting degraded Imperata grassland in Leyte (Philippines) made streamflow perennial again, we studied the hydrological behaviour of a 23‐year‐old mixed‐species “reforest” between June 2013 and May 2014. Typhoon Haiyan, one of the strongest events ever, caused major damage to the site in November 2013. Average daily apparent water use (ET) was 5.0 mm day−1 pretyphoon and 3.2 mm day−1 after disturbance. Corresponding ratios of period total quickflow Qq to precipitation were 16% and 44%. Quickflow volume and peak discharge increased rapidly once a threshold value of ~250 mm for soil water storage in the top 60 cm was exceeded. Before disturbance, quickflow consisted predominantly of lateral subsurface flow due to high soil hydraulic conductivities down to 60 cm. After disturbance, shallow groundwater rose regularly to within 10 cm of the surface on foot slopes, and saturation overland flow was observed during several large storms. Comparing estimated annual ET and Qq for undisturbed conditions for the reforest and a nearby degraded Imperata grassland microcatchment suggested that the extra infiltration following reforestation (~240 mm year−1) exceeded the extra ET by the reforest (100–185 mm year−1), implying a net positive trade‐off (55–140 mm year−1) and tentatively confirming local claims of improved dry‐season flow

Runoff response and sediment yield of a landslide-affected fire-climax grassland micro-catchment (Leyte, the Philippines) before and after passage of typhoon Haiyan

Decades of logging and slash-and-burn agriculture have turned vast tracts of land in tropical South-east Asia into unproductive fire-climax grasslands whose hydrological functioning is poorly known. To help fill this knowledge gap, a 3.2 ha landslide-affected Imperata grassland micro-catchment with perennial flow on Leyte Island (Philippines) was instrumented and monitored for a year. The area was hit by typhoon Haiyan on 8 November 2013, one of the largest events on record. Landslide surfaces covered 3.4% of the catchment prior to typhoon Haiyan and contributed to ‘direct runoff’ (Qq). This basic ‘contributing area’ increased to 7.7% by activation of old landslides and formation of new ones during typhoon Haiyan. Median storm runoff coefficients (Qq/P) based on straight-line hydrograph separation were 9% and 23% before (48 events) and after the typhoon (43 events), respectively, but the ratios of period-total Qq and P were much larger (24% and 47%, respectively). Both storm runoff volumes and peak discharge increased rapidly once a mid-slope water storage threshold for the upper 60 cm of soil of 250 mm was exceeded. Storm runoff contributions above those generated on landslides were most likely in the form of overland flow given the prevailing very low soil hydraulic conductivities. Post-typhoon water use of the heavily disturbed vegetation was reduced initially by nearly 70%, recovering to nearly 80% of the pre-typhoon value after ∼3 months. The high annual sediment yield (∼27 t ha−1) was heavily dominated by post-Haiyan sediment transport (94%); bedload contributed ∼8% of the total sediment yield

Soil physical characteristics of a degraded tropical grassland and a ‘reforest’: Implications for runoff generation

Imperata grassland soils are widely perceived as having poor physical and chemical properties that render them unproductive and prone to erosion. They are therefore increasingly targeted for reforestation across the tropics. To better understand how reforestation and forest growth affect soil hydrological processes we compared the soil physical characteristics for an Imperata grassland and a 23-year-old ‘reforest’ on Leyte Island, the Philippines. Saturated hydraulic conductivity (Ksat) was determined in the field (Amoozemeter, 20–90 cm depth) and the laboratory (small-core permeametry). Core-based values of Ksat were (much) lower than field-based values, suggesting macropores were not sampled adequately with the small cores. Ksat decreased exponentially with depth in both land-cover types, with a median field-measured Ksat of the grassland of 2.1 mm h−1 at the surface and 2.9 mm h−1 at 20–40 cm depth, declining to ≤1 mm h−1 below 60 cm. Corresponding values for the reforest were 59 (at 20 cm), 37 (at 40 cm) and 7.3 mm h−1 (at 60–100 cm depth). Reforest Ksat-values down to 60 cm depth were significantly higher than corresponding values in the grassland, but the difference disappeared at 90 cm depth. Organic carbon content in the top 40 cm of soil was slightly higher in the reforest than the grassland. Bulk density was higher and porosity marginally lower in the grassland than the reforest at all depths considered. The median five-min rainfall intensity (June 2013–May 2014) was 3.2 mm h−1, suggesting that >50% of the rainfall might generate Hortonian overland flow in the grassland. Overland flow is unlikely in the reforest where lateral flow is likely to be generated around 60 cm depth for ~30% of rain-time, versus 2–3% between 20 and 60 cm. Within the limitations of the space-for-time substitution approach, these results suggest that 23 years of forest development at Manobo had a positive effect on hillslope hydrological functioning

Soil water- and overland flow dynamics in a tropical catchment subject to long-term slash-and-burn agriculture

Years of slash-and-burn activities across the tropics have led to very a patchy land-cover with vegetation in various stages of regrowth but, the associated effects on runoff generation remain under-studied. We analysed soil moisture-, perched water level- and overland flow (OF) dynamics during two periods (15 February–2 November 2015 and 20 December 2015–2 March 2016) for plots in a small catchment in Eastern Madagascar where slash-and-burn agriculture has been practiced for more than 50 years: a 1.58 ha tree fallow (TF2), a 1.93 ha terraced shrub fallow (TSF), and a 0.08 ha degraded grassland plot with regularly coppiced and burned eucalypt trees (EUC). Near-surface saturated soil hydraulic conductivity (Ksat) was distinctly lower beneath TF2 compared to TSF and EUC, leading to distinctly different perched water level responses and OF occurrence. OF was highest for TF2 and lowest for TSF. Soil moisture content was lowest for EUC, resulting in the lowest antecedent moisture plus precipitation threshold for OF occurrence (82 mm compared to 129 mm for TSF and 137 mm for TF2). OF was generally in the form of saturation overland flow (SOF) and reflected perched water level dynamics, except at EUC where the occurrence of a perched water level was rare during the first measurement period. Soil moisture responses to rainfall at EUC were highly variable and became larger after harvesting and burning the plot prior to the second measurement period. These results show that soil physical properties and runoff-generation processes in areas with a long history of slash-and-burn agriculture can vary markedly over small spatial scales and need to be taken into account if catchment scale runoff responses are to be simulated or predicted