HS5.1.3

In many tropical areas slash-and-burn agriculture is an important driver of forest loss. In areas where slash-and-burn agriculture has been practiced for decades, land cover is typically a mosaic of patches of remnant forest, fields under active cultivation, fallows in various stages of regrowth (ranging from young shrub to semi-mature), and degraded fire-climax grasslands. Although runoff generation mechanisms are expected to be different for these different patches, little quantitative information is available in this regard, particularly at the catchment scale and over longer time-scales (i.e., multiple slash-and-burn cycles).

We re-instrumented a 31 ha catchment in upland Eastern Madagascar, where slash-and-burn agriculture has been practiced for more than 70 years in 2015; it had been monitored between 1963 and 1972 as well¹. We measured streamflow at two locations and overland flow and soil moisture for four hillside plots (0.05 – 1.93 ha): one plot under repeatedly coppiced and burned Eucalyptus and three plots under young shrub and tree fallows. One of the plots underwent rudimentary terracing in the past. We analysed the rainfall-runoff dynamics for 50 rainfall events (median 12 mm, maximum 71 mm).

For 60% of the events, the stormflow coefficient (minimum contributing area) was <3%, which is the proportion of valley-bottom wetlands and rice paddies in the catchment. Stable isotope sampling for five storm runoff events indicate a maximum total event-water contribution of 16%. However, instantaneous event-water contributions were as high as 66%. The hillside plot runoff response was dominated by saturation-excess overland flow and showed strong threshold behaviour in terms of the antecedent soil moisture storage in the upper 30 cm of the soil plus the event total rainfall amount (ASI + P). Average threshold values for overland flow occurrence ranged from 87 mm for the coppiced Eucalyptus to 137 mm for the young fallow plots (regardless of terrace presence). Stormflow also increased after an ASI+P-threshold was exceeded (100 mm based on the soil moisture sensors for the Eucalyptus plot and 150 mm for the sensors at the tree fallow plots).

These results indicate an increased hydrological connectivity between hillslopes and valley bottom under wetter conditions and that stormflow in the study catchment is strongly affected by variations in seasonal rainfall. The results will be used to validate a hydrological model to determine the net effect of concurrent changes in soil infiltrability and vegetation water use associated with forest loss and recovery on stormflow totals and the seasonal flow regime.

¹Bailly, C., de Coignac, G.B., Malvos, C., Ningre, J.M., and Sarrailh, J.M. (1974). Étude de l'influence du couvert naturel et de ses modifications á Madagascar. Expérimentations en bassins versants élémentaires. Cahiers Scientifiques, 4. Centre Scientifique Forestier Tropical, Nogent-sur-Marne, France, 114 pp.

How to cite: Zwartendijk, B. W., van Meerveld, H. J. (., Teuling, R. J., Ghimire, C. P., and Bruijnzeel, L. A.: Dominant drivers of runoff in a slash-and-burn affected catchment in upland Eastern Madagascar, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3957, https://doi.org/10.5194/egusphere-egu21-3957, 2021.

Proactive thinning and controlled burning are being utilized to mitigate the effects of severe wildfires across the globe. Hydrologic function of watersheds after wildfire and clear-cutting has been well documented, however the impacts of pre-fire mitigation strategies are less understood. The current study utilized two mixed precipitation watersheds, which supply drinking water for Ashland, Oregon, USA, to assess the effectiveness of restoration and fuel reduction strategies on hydrologic change. This Mediterranean dry mixed conifer-hardwood habitat is unique as it sits in the convergence point of several ecoregions, providing significant biological diversity for the region. Hydrologic response from prior mitigation strategies was evaluated using max monthly flow, mean annual 7-day low flow, runoff ratios, timing and total water yield. Results show an average decrease of 26% and 24% in total annual water yields in the West and East basins of the Ashland watershed, respectively. Analysis also showed that 66% (West) and 72% (East) of the changes in water yield were due to annual variations in precipitation, demonstrating that land cover changes were not the dominant driver of hydrologic change. Current work includes identifying the thresholds at which stand density reduction leads to an increase in annual surface water yield. The integrated surface and groundwater model, MIKE SHE, is developed and used to simulate a range of forest fire mitigation efforts based upon representative parameters in the model, including leaf area index. Findings will then be expanded to include stand density index for better interpretation of our findings to make recommendations for local and regional forest managers. Ultimately, results will help inform future implementation of forest restoration and climate adaptation at larger scales.

How to cite: Kurzweil, J., Abdi, R., Metlen, K., and Hogue, T.: Understanding the hydrologic impacts of wildfire management strategies using MIKE SHE, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8860, https://doi.org/10.5194/egusphere-egu21-8860, 2021.

Land Use Land Cover (LULC) change is widely recognised as one of the most important factors impacting river basin hydrology.  It is therefore imperative that the hydrological impacts of various LULC changes are considered for effective flood management strategies and future infrastructure decisions within a catchment.  The Soil and Water assessment Tool (SWAT) has been used extensively to assess the hydrological impacts of LULC change.  Areas with assumed homogeneous hydrologic properties, based on their LULC, soil type and slope, make up the basic computational units of SWAT known as the Hydrologic Response Units (HRUs).  LULC changes in a catchment are typically modelled by SWAT through alterations to the input files that define the properties of these HRUs.  However, to our knowledge at least, the process of making such changes to the SWAT input files is often cumbersome and non-intuitive.  This affects the useability of SWAT as a decision support tool amongst a wider pool of applied users (e.g., engineering teams in environmental regulatory agencies and local authorities).  In this study, we seek to address this issue by developing a user-friendly toolkit that will: (1) allow the end user to specify, through a Graphical User Interface (GUI), various types of LULC changes at multiple locations within their study catchment, (2) run the SWAT+ model (the latest version of SWAT) with the specified LULC changes, and (3) enable interactive visualisation of the different SWAT+ output variables to quantify the hydrological impacts of these scenarios.  Importantly, our toolkit does not require the end user to have any operational knowledge of the SWAT+ model to use it as a decision support tool.  Our toolkit will be trialled at 15 catchments in Gwynedd county, Wales, which has experienced multiple occurrences of high flood events, and consequent economic damage, in the recent past.  We anticipate this toolkit to be a valuable addition to the decision-making processes of Gwynedd County Council for the planning and development of future flood alleviation schemes as well as other infrastructure projects.

How to cite: Rigby, A., Patil, S., and Ritsos, P.: A novel toolkit to streamline Land Use Land Cover change assessment in the SWAT+ model to enhance flood management and infrastructure decisions, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4139, https://doi.org/10.5194/egusphere-egu21-4139, 2021.

Rapid human-induced changes, such as climate change, population growth and rapid urbanization, are putting enormous stress on water resources. An accurate estimate of available water resources is a prerequisite for sustainable water resources planning and management. For gauged basins, historical records of hydrological observations are available, but for ungauged basins, the assessment of water availability is a challenging task. Therefore, the major focus of studies in ungauged basins is the development of appropriate tools that can accurately quantify hydrologic responses under various land use and climatic conditions. The reduction of the number of unknown parameters to be estimated is a key aspect in the development of hydrological models for ungauged basins.

This work is part of these issues and proposes an approach to reduce the complexity of hydrological models that include substantial uncertainties about the input data, initial and boundary conditions, model structure and parameters, owing to lack of data (i.e. for ungauged basins) and poor knowledge of hydrological response mechanisms. The case study of a basin of the District of Licola, located in the territory of the municipality of Giugliano, a city near Naples (southern Italy) is analyzed. Originally devoted to agriculture and grazing, it has been affected in the last decades by intense urbanization, which caused an increase in the impermeability of the soil cover. The increase in residential, commercial and production buildings has changed the functioning of the drainage network canals, compared to the original conditions, causing an increase in the frequency of flooding in the area. The semi-distributed hydrological model SWMM is adopted, which allows the subdivision of the basin in sub-basins according to land use and soil data.

Sensitivity Analysis (SA) is an effective approach to model simplification, providing an assessment of how much each input / parameter contributes to the output uncertainty. In general, SA is an essential part of model development, reducing uncertainties that have negative effects on the accuracy and reliability of simulated results. Specifically, in this study the SA is carried out with a method based on the decomposition of the variance of the peak flow and runoff volume, to quantitatively evaluate the contributions of single uncertain inputs/parameters that characterize the surface runoff with respect to different rainfall events, for both pervious and impervious areas. To this aim, the Fourier Amplitude Sensitivity Test (FAST) is implemented. This method allows quantifying not only the “main effect” of variance, but also provides the Total Sensitivity Indices (TSI), defined as the sum of all the sensitivity indices for each parameter (including the effects of the interaction with other uncertain parameters).

The research objectives aims at: (i) increased understanding of the relationships between input and output variables in a complex hydrological system; (ii) reduction of model uncertainty, through the identification of input parameters mostly contributing to output variability and should therefore be the focus of sensitivity analysis; (iii) model simplification, fixing  the values of input parameters that have little effect on the output, and identifying and removing redundant parts of the model structure.

How to cite: Di Cicco, I., Giudicianni, C., Di Nardo, A., and Greco, R.: Sensitivity analysis of surface runoff parameters for hydrological modeling of periurban ungauged basin , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13249, https://doi.org/10.5194/egusphere-egu21-13249, 2021.

Changes in hydrological regime associated with land cover change may result in crucial implications to tropical peatland landscape since hydrology strongly controls peatland geomorphology, ecology, and biogeochemical cycle. Therefore, improved understanding of the land cover change impacts to the water balance is of significant importance in order to formulate responsible peatland management strategies. In this study, we investigated the water balance under historical land cover change within Padang Island, Indonesia, an ombrotropic tropical peatland landscape with heterogeneous land covers. For this purpose, we established a model setup using a coupled MIKE SHE and MIKE Hydro River. The model was calibrated and validated against comprehensive data set from field measurements. Land cover change impacts were evaluated by comparing the water balance under current and past condition. The past land cover distribution was derived from historical satellite imagery analysis covering the period of 25 years before the current condition. Meanwhile, the past topography data was generated following long-term subsidence monitoring data. Here, we will present the impacts of land cover change to water balance at the landscape level and their implications for management of tropical peatlands.

How to cite: Asyhari, A., Kurnianto, S., Suardiwerianto, Y., Tanjungsari, R. J., Hidayat, M. F., Harahap, M. I. F., Marpaung, S. M., Salam, Y. W., and Deshmukh, C. S.: Tropical Peatland Water Balance under Land Cover Change in Padang Island, Indonesia, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7229, https://doi.org/10.5194/egusphere-egu21-7229, 2021.

Nowadays, one major issue concerns the land-use change due to urban developments that alters the basin response to meteorological events. With less storage capacity and more rapid runoff, urban river basins rise more quickly during storms and have higher peak discharge rates than rural catchments.

An exemplary case of this situation is the city of Milan and its whole territory that extends towards north that collects meteoric precipitation, through the Seveso, Olona and Lambro (SOL) rivers plus a number of minor tributaries for a total drainage surface of about 1300 km².

In order to assess the impact of anthropogenic development on urban catchment scale hydrology, a reanalysis of 40 years of simulations has been carried out with the Curve Number (CN) map based on current land use, and compared to simulations with the CN maps derived using past land use.

A coupled hydro-meteorological system which comprises the physically based rainfall-runoff hydrological model FEST-WB, developed by the Politecnico di Milano and the ERA5-Land hourly dataset from 1981 to present, provided by ECMWF under the framework of Copernicus Climate Change Service Programme has been built.

The study (named as SOL40) exactly analyses 40-years trends of the main meteorological (air temperature, precipitation, etc.) and hydrological variables (soil moisture, evapotranspiration and runoff) over the SOL area, and try to quantify and separate the impact of land use change from the climate change scenario.

How to cite: Ceppi, A., Gambini, E., Lombardi, G., Ravazzani, G., and Mancini, M.: SOL40: forty years of simulations under a climate and land use change scenario, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12496, https://doi.org/10.5194/egusphere-egu21-12496, 2021.

Land use/land cover (LULC) change is a major factor affecting the hydrological response at the watershed scale. However, hydrological modelling, in its current practice, is usually carried using a single and static LULC layer for simulation runs over long periods. Eventually, this approach leads to failure in accounting for LULC spatial and temporal changes as well as non-linear impacts on simulated outputs. Besides, in the typical case of Sahelian hydrosystems, previous modelling attempts based on this approach failed at reproducing the well-known Sahelian hydrological paradox which occurred in the area during the period 1970-1990. This study aims at assessing the added value of dynamical integration of LULC changes in hydrological modelling of surface runoff in Sahelian hydrosystems. The Tougou watershed (37 km²), located in Northern Burkina Faso is selected as a case study. LULC maps of the watershed are produced from 1952 to 2017 from the processing of Landsat satellite images. The SWAT (Soil & Water Assessment Tool) model, using the SCS-CN method (for surface runoff estimation), is calibrated and validated using observed runoff data collected over the period 2004-2018. The calibration/validation is carried using LULC maps of the watershed in 1999, 2009 and 2017, dynamically integrated into the model using a specific land use update module. Further, the calibrated model parameters helped in the reconstitution of surface runoff over the historical period 1952-2005 and integrating dynamically LULC maps in 1952, 1973, 1986 and 1999. The results showed that between the periods 1952-1968 (P1) and 1986-2005 (P3), the average annual rainfall decreased by 87.9 mm while paradoxically, average annual runoff increased by 1 mm. Further analysis revealed that the increase in runoff is mainly attributed to LULC changes (+647%) which offsets the effect of the decrease in rainfall (-547%). From the analysis of LULC maps, it was found that from P1 to P3 periods, the decrease in natural vegetation (CN = 67.3 ± 5.7) by 40%, replaced by bare and degraded soils (CN = 83.8 ± 2.5) explained the observed increase in surface runoff potential of the watershed, as shown by their calibrated CN values. These findings are reminiscent of the Sahelian hydrological paradox reported in the literature and provide evidence of the sensitivity of surface runoff to LULC changes. Overall, the results call to hydrologists, water resources planners and managers, regarding the advantages of coupling LULC changes in hydrological modelling. Also, the study advocates for the development of integrated modelling platforms integrating both LULC changes and hydrological modelling to allow a better understanding and the more accurate long-term forecasting of water resources, in particular in the case of Sahelian hydrosystems.

Keywords: Dynamic LULC input, Hydrological modelling, Surface runoff, SWAT model, Burkina Faso, Sahelian paradox.

How to cite: Yonaba, R., Biaou, A. C., Koita, M., Fowé, T., Mounirou, A. L., Zouré, C. O., Queloz, P., Karambiri, H., and Yacouba, H.: Hydrological modelling of Sahelian hydrological paradox: accounting for explicit land use/land cover change in the simulation of hydrological processes, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7974, https://doi.org/10.5194/egusphere-egu21-7974, 2021.

Redistribution of evapotranspiration from land via atmospheric circulation is an important Earth system process. Globally, evapotranspiration contributes significantly to terrestrial rainfall, on both regional and more remote scales. In wet, tropical regions (e.g. the Congo basin), transpiration and interception loss from the dense forest cover are the primary drivers of moisture recycling, which plays a crucial role in preserving regional ecosystem functioning. However, for semi-arid and arid regions, our knowledge on the extent and significance of evapotranspiration for moisture recycling is still very limited, despite the significance this may have for addressing challenges of desertification in times of rapid environmental change. Considering this, we are taking the Sahel region as a case study and investigate its contribution to precipitation in the African continent. In addition, we specifically study what fraction of the precipitation originates from vegetation in the Sahel through transpiration and interception loss. Our study is based on simulated atmospheric moisture trajectories derived from the Lagrangian model FLEXPART with a 1-degree resolution, driven by ECMWF reanalysis data over 1980–2016. Preliminary results show (1) the temporal variability in the contribution of the region to precipitation in African drylands, and (2) a significant contribution of local precipitation recycling. We conclude that consideration of the naturally and anthropogenically-driven greening of the Sahel, as well as land use and land cover changes in the region, may have both local and far-reaching impacts via the transport of moisture through the atmosphere.

How to cite: te Wierik, S., Keune, J., Miralles, D., Cammeraat, E., Artzy-Randrup, Y., and Gupta, J.: Contribution of the Sahel region to precipitation over the African continent, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15415, https://doi.org/10.5194/egusphere-egu21-15415, 2021.

As in other parts of the Indian subcontinent, the forests of Meghalaya (NE India) provide an array of environmental services but the prevalence of traditional slash-and-burn agriculture (locally called jhum) plus mining has led to severely degraded hillsides and a critical loss of soil water storage opportunity and groundwater recharge. As a result, despite receiving some of the highest rainfall totals in the world (MAP up to 11 m, 75% received between May and September), the Meghalaya plateau faces severe water scarcity during the five-month long dry season. In response to such problems, initiatives have been taken towards restoring hydrological functioning through reforestation and assisted natural regeneration (ANR) programmes. As a first step towards assessing the possible improvement of soil physical characteristics and associated hydrological functioning after several years of ANR we measured topsoil (0–10 cm) saturated hydraulic conductivity (K_sat) using double-ring infiltrometry at 12 sites in the Khasi Hills that represented three contrasting vegetation covers: (i) sacred forest (n = 6, natural baseline), (ii) 2–10-year-old ANR (n = 3), and (iii) degraded Imperata grassland (n = 3, degraded reference). At each site, nine K_sat-measurements were taken along the hillslope gradient. In addition, at three sites, blue dye infiltration experiments (n = 2 per site) were carried out to examine the dominant percolation pathways associated with each land-cover type. The median K_sat value for the sacred forest sites was highest (373 mm h^-1), reflecting the abundance of biologically mediated macropores arising from the decomposing activity of soil microflora and fauna at these relatively undisturbed sites. The corresponding value for the ANR sites (160 mm h^-1) was much higher than the median K_sat for the degraded grasslands (71 mm h^-1) but still considerably below the forest reference. Limited observations of topsoil bulk density and carbon content (n = 5 samples in each of three plots) showed increasing bulk density and decreasing carbon content from forest via ANR to grassland, thereby reflecting the observed trend in K_sat. The blue dye experiments suggested infiltration in the sacred forest was dominated by flow along roots and other preferential flow pathways whereas the degraded grassland was mostly characterized by matrix flow. The ANR site showed intermediate behaviour with macropore flow exhibiting high matrix interaction. Comparison of observed median topsoil K_sat in top-layer with prevailing (maximum) hourly rainfall intensities for Cherrapunji suggested infiltration-excess overland flow (IOF) must be considered a rare phenomenon in the sacred forest. Conversely, the K_sat-values for the ANR and degraded grassland sites indicated the occurrence of IOF at high-intensity rainfall events. Despite the observed improvement in surface K_sat it cannot be excluded that the generally shallow nature and high stoniness of the soils pose serious limitations to rebuilding soil water storage capacity through ANR/reforestation. Furthermore, frequent occurrence of saturation-excess OF at the height of the monsoon and associated surface erosion cannot be excluded.

How to cite: Ghimire, C. P., Zwartendijk, B. W., Pde, F., and Bruijnzeel, L. A.: Changes in soil hydraulic conductivity and preferential flow pathways after assisted forest restoration on degraded land in the Khasi Hills (Meghalaya, NE India), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6559, https://doi.org/10.5194/egusphere-egu21-6559, 2021.

The aim of this study is the assessment of the effect of riparian vegetation management practices on the hydrodynamic behaviour of vegetated water bodies colonized by riparian weed species. The present study was carried out by calibrating the most suitable literature models on flow resistance of vegetated flows, by considering different riparian vegetation management scenarios. The key dimensional features of the examined riparian vegetation elements were estimated by comparing the outcomes of the digital processing of remote sensing and proximity sensors extremely widespread in both agricultural and forestry studies and applications. The main remarks of this study well demonstrated that the knowledge of the hydraulic and hydrological proprieties of vegetated flows at field scale is sensibly affected by the uncertainties in the measurements of riparian vegetation biomass and water quality, differently from experimental analyses conducted in dedicated flumes in the laboratory performed in standard environment conditions.

How to cite: Crimaldi, M. and Lama, G. F. C.: Assessing the impact of land use management on the hydrodynamic behavior of vegetated water bodies colonized by riparian weed species, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12842, https://doi.org/10.5194/egusphere-egu21-12842, 2021.

Changes in land use and land cover (LULC) can have direct effects on streamflows at the catchment scale. However, studies of this type are still lacking in some tropical areas of the globe where LULC changes constitute the main antropic changes at watershed scale; for instance, deforestation, and agricultural modification among others. Therefore, for analysing impacts on streamflow, the first challenge is to achieve a good characterization of LULC. 

Here the objective is to evaluate whether these changes have occurred for two small basins on the South American coast in Ecuador (Junín and Paján) and, to analyse their possible effects on flows with special emphasis on minimum flows. Both catchments are characterized by savanna (Aw) climate with  seasonal rainfall patterns and a long period of minimum to zero precipitation. Changes in LULC are studied using two sources of remote sensing information: ESA-CCI-LC at spatial resolution of 300 m and Landsat TM at spatial resolution of 30 m, together with local authorities reports within the last decades. While ESA-CCI-LC directly provides LULC information , LULC was retrieved from surface reflectance after preprocessing (atmospheric and topographic corrections) from Landsat by  using a supervised algorithm. In a preliminary approach the observed changes are compared with variation in streamflow in the outlet of both catchment. 

Results show that for one of the catchments, Junín, ESA-CCI-LC does not reflect changes, while the analysis with Landsat TM shows a decrease of 11.7% of evergreen forest and an increase in agricultural activities. For the other basin, Paján, the first source indicates an increase in evergreen forest, while the second source indicates the opposite, a decrease of 7.4%. The contradictions in these sources highlight the importance of taking into account local knowledge as well as the appropriate selection of spatial resolution in the analysis. Finally, regarding the effects of the LULC changes in streamflow the initial approach was not sufficient to capture any direct effect and therefore, a deeper analysis based on specific features of the streamflow signal are planned as the next step.  

How to cite: Giler-Ormaza, A., Pimentel, R., and Aguilar, C.: Assessing changes in land use and their effects on river flows in tropical catchments of Ecuador, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13190, https://doi.org/10.5194/egusphere-egu21-13190, 2021.

Forests worldwide supply moisture to downwind precipitation through moisture recycling. Agricultural areas located downwind of forests are, hence, susceptible to changes in precipitation caused by upwind forest changes. In fact, human activities have driven extensive forest cover changes in different parts of the world, in different directions, and at different rates. Nevertheless, the forest-agriculture relationship has yet to be systematically quantified and mapped globally. Previous regional studies in South America show that upwind deforestation of the Amazon forest can reduce downwind precipitation and thus decrease agricultural production. A global coverage analysis of forest-agriculture relationship is therefore necessary to identify other hotspot regions where downwind agriculture relies heavily on upwind forests. In this study, we establish the global source-to-sink relationship between forests and their downwind agriculture by analysing 10 years of high resolution (0.25°x0.25°) ERA5-based moisture flows processed by the UTrack moisture tracking model. We assess the seasonality of the reliance on forests considering the growing season of crops cultivated in the downwind regions. Our study provides a global overview of the cross-sectoral and remote dependence of agriculture on forests globally through moisture recycling.

How to cite: Pranindita, A., Teuling, A. J., Fetzer, I., and Wang-Erlandsson, L.: The role of forests in securing water for agriculture globally, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15766, https://doi.org/10.5194/egusphere-egu21-15766, 2021.