Abstract. Adapting and improving the hydrological processes in land surface models are crucial given the increase in the resolution of the climate models to correctly represent the hydrological cycle. The present paper introduces a floodplain scheme adapted to the higher-resolution river routing of the Organising Carbon and Hydrology In Dynamic Ecosystems (ORCHIDEE) land surface model. The scheme is based on a sub-tile parameterisation of the hydrological units – a hydrological transfer unit (HTU) concept – based on high-resolution hydrologically coherent digital elevation models, which can be used for all types of resolutions and projections. The floodplain scheme was developed and evaluated for different atmospheric forcings and resolutions (0.5∘ and 25 km) over one of the world's largest floodplains: the Pantanal, located in central South America. The floodplain scheme is validated based on the river discharge at the outflow of the Pantanal which represents the hydrological cycle over the basin, the temporal evolution of the water mass over the region assessed by the anomaly of total water storage in the Gravity Recovery And Climate Experiment (GRACE), and the temporal evaluation of the flooded areas compared to the Global Inundation Extent from Multi-Satellites version 2 (GIEMS-2) dataset. The hydrological cycle is satisfactorily simulated; however, the base flow may be underestimated. The temporal evolution of the flooded area is coherent with the observations, although the size of the area is underestimated in comparison to GIEMS-2. The presence of floodplains increases the soil moisture up to 50 % and decreases average temperature by 3 ∘C and by 6 ∘C during the dry season. The higher soil moisture increases the vegetation density, and, along with the presence of open-water surfaces due to the floodplains, it affects the surface energy budget by increasing the latent flux at the expense of the sensible flux. This is linked to the increase in the evapotranspiration related to the increased water availability. The effect of the floodplain scheme on the land surface conditions highlights that coupled simulations using the floodplain scheme may influence local and regional precipitation and regional circulation.
Abstract. Adapting and improving the hydrological processes in Land Surface Models is crucial given the increase of the resolution of the Climate Models to correctly represent the hydrological cycle. The present paper introduces a floodplains scheme adapted to the higher resolution river routing of the ORCHIDEE Land Surface Model. The scheme is based on a sub-tile parameterization of the hydrological units, Hydrological Transfer Unit concept (HTUs), based on high resolution hydrologically-coherent Digital Elevation Models which can be used for all types of resolutions and projections. The floodplain scheme was developed and evaluated for different atmospheric forcings and resolutions (0.5° and 25 km) over one of the world’s largest floodplains: the Pantanal, located in Central South America. The floodplains scheme is validated based on the river discharge at the outflow of the Pantanal which represents the hydrological cycle over the basin, the temporal evolution of the water mass over the region assessed by the anomaly of Total Water Storage in Gravity Recovery And Climate Experiment (GRACE) and the temporal evaluation of the flooded areas compared to the Global Inundation Extent from Multi-Satellites dataset (GIEMS-2). The hydrological cycle is satisfactorily simulated, however, the base flow may be underestimated. The temporal evolution flooded area is coherent with the observations although the size of the is underestimated in comparison to GIEMS-2. The presence of floodplains increases the soil moisture up to 50 % and decreases average temperature with 3 °C and with 6 °C during the dry season. The higher soil moisture increases the vegetation density and, along with the presence of open water surfaces due to the floodplains, it affects the surface energy budget by increasing the latent flux at the expense of the sensible flux. This is linked to the increase of the evapotranspiration related to the increased water availability. The effect of the floodplains scheme on the land surface conditions highlights that coupled simulations using the floodplains scheme may influence local and regional precipitation and regional circulation.
Abstract. Land surface models (LSMs) use the atmospheric grid as their basic spatial decomposition because their main objective is to provide the lower boundary conditions to the atmosphere. Lateral water flows at the surface on the other hand require a much higher spatial discretization as they are closely linked to topographic details. We propose here a methodology to automatically tile the atmospheric grid into hydrological coherent units which are connected through a graph. As water is transported on sub-grids of the LSM, land variables can easily be transferred to the routing network and advected if needed. This is demonstrated here for temperature. The quality of the river networks generated, as represented by the connected hydrological transfer units, are compared to the original data in order to quantify the degradation introduced by the discretization method. The conditions the sub-grid elements impose on the time step of the water transport scheme are evaluated, and a methodology is proposed to find an optimal value. Finally the scheme is applied in an off-line version of the ORCHIDEE (Organising Carbon and Hydrology In Dynamic Ecosystems) LSM over Europe to show that realistic river discharge and temperatures are predicted over the major catchments of the region. The simulated solutions are largely independent of the atmospheric grid used thanks to the proposed sub-grid approach.
Abstract. Land Surface Models (LSMs) use the atmospheric grid as their basic spatial decomposition because their main objective is to provide the lower boundary conditions to the atmosphere. Lateral water flows at the surface on the other hand require a much higher spatial discretization as they are closely linked to topographic details. We propose here a methodology to automatically tile the atmospheric grid into hydrological coherent units which are connected through a graph. As water is transported on sub-grids of the LSM, land variables can easily be transferred to the routing network and advected if needed. This is demonstrated here for temperature. The quality of the river networks generated, as represented by the connected hydrological transfer units, are compared to the original data in order to quantify the degradation introduced by the discretization method. The conditions the sub-grid elements impose on the time step of the water transport scheme are evaluated and a methodology is proposed to find an optimal value. Finally the scheme is applied in an off-line version of the ORCHIDEE LSM over Europe to show that realistic river discharge and temperatures are predicted over the major catchments of the region. The simulated solutions are largely independent of the atmospheric grid used thanks to the proposed sub-grid approach.
Abstract. Adapting and improving the hydrological processes in Land Surface Models is crucial given the increase of the resolution of the Climate Models to correctly represent the hydrological cycle. The present paper introduces a floodplains scheme adapted to the higher resolution river routing of the ORCHIDEE Land Surface Model. The scheme is based on a sub-tile parameterization of the hydrological units, Hydrological Transfer Unit concept (HTUs), based on high resolution hydrologically-coherent Digital Elevation Models which can be used for all types of resolutions and projections. The floodplain scheme was developed and evaluated for different atmospheric forcings and resolutions (0.5° and 25 km) over one of the world’s largest floodplains: the Pantanal, located in Central South America. The floodplains scheme is validated based on the river discharge at the outflow of the Pantanal which represents the hydrological cycle over the basin, the temporal evolution of the water mass over the region assessed by the anomaly of Total Water Storage in Gravity Recovery And Climate Experiment (GRACE) and the temporal evaluation of the flooded areas compared to the Global Inundation Extent from Multi-Satellites dataset (GIEMS-2). The hydrological cycle is satisfactorily simulated, however, the base flow may be underestimated. The temporal evolution flooded area is coherent with the observations although the size of the is underestimated in comparison to GIEMS-2. The presence of floodplains increases the soil moisture up to 50 % and decreases average temperature with 3 °C and with 6 °C during the dry season. The higher soil moisture increases the vegetation density and, along with the presence of open water surfaces due to the floodplains, it affects the surface energy budget by increasing the latent flux at the expense of the sensible flux. This is linked to the increase of the evapotranspiration related to the increased water availability. The effect of the floodplains scheme on the land surface conditions highlights that coupled simulations using the floodplains scheme may influence local and regional precipitation and regional circulation.
Abstract. Adapting and improving the hydrological processes in Land Surface Models is crucial given the increase of the resolution of the Climate Models to correctly represent the hydrological cycle. The present paper introduces a floodplains scheme adapted to the higher resolution river routing of the ORCHIDEE Land Surface Model. The scheme is based on a sub-tile parameterization of the hydrological units, Hydrological Transfer Unit concept (HTUs), based on high resolution hydrologically-coherent Digital Elevation Models which can be used for all types of resolutions and projections. The floodplain scheme was developed and evaluated for different atmospheric forcings and resolutions (0.5° and 25 km) over one of the world’s largest floodplains: the Pantanal, located in Central South America. The floodplains scheme is validated based on the river discharge at the outflow of the Pantanal which represents the hydrological cycle over the basin, the temporal evolution of the water mass over the region assessed by the anomaly of Total Water Storage in Gravity Recovery And Climate Experiment (GRACE) and the temporal evaluation of the flooded areas compared to the Global Inundation Extent from Multi-Satellites dataset (GIEMS-2). The hydrological cycle is satisfactorily simulated, however, the base flow may be underestimated. The temporal evolution flooded area is coherent with the observations although the size of the is underestimated in comparison to GIEMS-2. The presence of floodplains increases the soil moisture up to 50 % and decreases average temperature with 3 °C and with 6 °C during the dry season. The higher soil moisture increases the vegetation density and, along with the presence of open water surfaces due to the floodplains, it affects the surface energy budget by increasing the latent flux at the expense of the sensible flux. This is linked to the increase of the evapotranspiration related to the increased water availability. The effect of the floodplains scheme on the land surface conditions highlights that coupled simulations using the floodplains scheme may influence local and regional precipitation and regional circulation.
<p>The evolution and possible limitation of water resources under climate change will become a crucial problem over the next decades and accurate hydrological projections are fundamental tools to assess the problem. The goal of this study is to improve the simulation of both river discharges and evaporation with the ORCHIDEE (Organising Carbon and Hydrology in Dynamic Ecosystems) land surface model by accounting for a high-resolution river network and water management influence.</p><p>This work will allow us to produce long-term projections of river discharge in France under different regional-scale climate change scenarios for the national project Explore2 and the French climate services.</p><p>To this end, we present here the evaluation and calibration of an improved version of ORCHIDEE, run off-line over France with atmospheric forcing from the SAFRAN reanalysis at an 8-km resolution and 1-hourly time step. First, we implement a high-resolution river routing scheme recently developed to better reproduce the water flow through the river network from the source to the outlet. It relies on topographical and hydrological information from the MERIT&#160;Hydro (Multi-Error-Removed Improved-Terrain) digital elevation model scaled at a 2km resolution,&#160;which&#160;allows us to define sub-basins at a higher resolution than the atmospheric forcing and to correctly position a majority of French gauging stations along the reconstructed rivers.</p><p>By comparing the discharge simulations to observations from the French hydrometric database (<span role="link">http://hydro.eaufrance.fr/</span>) on about 800 stations with variable upstream areas, selected for their long and good-quality record, and medium-to-low human pressures, we find a very general overestimation of river discharge by the model, except in mountainous areas where earlier studies showed that the SAFRAN reanalysis was underestimating precipitation. The comparison of the simulated evapotranspiration to the data-driven FLUXCOM gridded product, over the upstream area of each selected station, shows a systematic underestimation, which can be explained by the underestimation of precipitation over mountains, and is elsewhere consistent with the overestimation of river discharge.</p><p>Further comparison to water withdrawals and consumption data from the national database BNPE (<span role="link">http://bnpe.eaufrance.fr/</span>) suggests that both river discharge overestimate and evapotranspiration underestimate can be partly attributed to the neglect of water management in ORCHIDEE, although the studied stations have been selected for their weak human influence. We will thus incorporate water management information in ORCHIDEE&#160;in two ways: by activating an irrigation parametrization to consistently describe the impact of this human pressure on both river discharge and evapotranspiration, and by reducing river discharge from the other abstraction sources. The related parameters will finally be calibrated such as to best reproduce the observed discharge, evapotranspiration, and irrigation withdrawals.</p>
