Assessing the hillslope-channel contributions to the catchment sediment balance under land use and climate change

JPC Eekhout, A Jódar-Abellán, E Carrillo-López, C Boix-Fayos and J de Vente

July 24-25, 2024

ICCE, Eichstätt, Germany

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Abstract

Most soil erosion models simulate hillslope erosion processes, such as sheet and rill erosion. However, observations have shown that channel erosion processes become important at larger spatial scales, which are hardly ever considered in soil erosion impact assessments. Hence, to get a full understanding of the impacts of global change on the catchment-scale sediment balance, models are needed that combine hillslope soil erosion processes with channel morphodynamics. Here we present a modification to the SPHY-MMF model that includes a novel channel morphodynamics module, which determines erosion and deposition in rills and channels, besides sheet erosion that was previously implemented in the SPHY-MMF model. We applied the model to a Mediterranean study area in southeast Spain. The model was calibrated using observed check dam sediment yield data, which gave satisfactory results for check dams located in the channels. The model was subsequently applied under historical land use change and future climate change scenarios. The model simulations show that channel erosion contributes substantially (35%–40%) to the total sediment yield, highlighting the importance of accounting for channel erosion in catchment-scale sediment budget estimations. The land use change scenarios were applied in a smaller subcatchment, which is characterized by reforestation and check dam construction between 1956 and 2016. The results show an 80% decrease in sediment yield at the outlet, because of the implementation of these soil conservation measures. The results also show a slight increase of rill and channel erosion, which may be a response to the decrease in sediment input from the hillslopes. The climate change scenarios show that the different erosional processes (i.e. sheet, rill, channel) are projected to decrease or increase, depending on the projected change in annual and extreme precipitation. From this we conclude that interactions between different erosional and depositional processes should be considered when studying the impact of global change on the catchment-scale sediment balance. This will also allow policy makers to plan conservation measures more efficiently, by applying measures close to the sources with the highest contribution to the catchment-scale sediment yield, accounting for possible interactions with other erosion or deposition processes within the catchment.
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Slide 1 of Assessing the hillslope-channel contributions to the catchment sediment balance under land use and climate change
1/20
Today I present the results of a recently published study in which we implemented channel morphodynamics in the hydrology-soil erosion model SPHY-MMF.
Slide 2 of Assessing the hillslope-channel contributions to the catchment sediment balance under land use and climate change
2/20
I will first give a short introduction to the SPHY-MMF model, which is a distributed process-based hydrological model with a daily time step. It simulates the most relevant hydrological processes as shown in the figure here on the right. A few years ago we implemented infiltration excess surface runoff, which is especially relevant for Mediterranean environments where we usually apply the model.
Slide 3 of Assessing the hillslope-channel contributions to the catchment sediment balance under land use and climate change
3/20
A few years ago, we implemented the Morgan-Morgan-Finney soil erosion model into SPHY, hence the name SPHY-MMF. MMF simulates sheet and rill erosion and accounts for immediate deposition. We also implemented a sediment transport module to transport sediment towards the catchment outlet, while accounting for deposition in the channels.
Slide 4 of Assessing the hillslope-channel contributions to the catchment sediment balance under land use and climate change
4/20
Several studies have shown that large scale erosion processes, such as gully and channel erosion, may significantly account for the sediment yield at the catchment scale. The aim of this study was to implement channel processes into SPHY-MMF, to improve the applicability at large spatial scales.
Slide 5 of Assessing the hillslope-channel contributions to the catchment sediment balance under land use and climate change
5/20
The first step was to update the water routing algorithm of the model, which was initially a simple accumulated flux algorithm. To be able to simulate channel morphodynamics, we had to implement more advanced sediment transport equations that are forced by water depth or flow velocity, which the model did not account for.
Slide 6 of Assessing the hillslope-channel contributions to the catchment sediment balance under land use and climate change
6/20
We implemented a water routing based on a travel time algorithm. The algorithm is a function of channel storage and flow velocity. We implemented an iteration within the model code, with a discharge threshold as model parameter.
Slide 7 of Assessing the hillslope-channel contributions to the catchment sediment balance under land use and climate change
7/20
The new water routing algorithm requires more input data then before. This includes the channel dimensions, including channel width and depth and floodplain width. You need to assign a Manning’s value for the channel and floodplain. And a map indicating the hillslopes and the channels.
Slide 8 of Assessing the hillslope-channel contributions to the catchment sediment balance under land use and climate change
8/20
We implemented two types of sediment transport equations, i.e. different equations for rills and for channels. In this study we used the Govers and Yalin equations, but the other ones are optional, as well.
Slide 9 of Assessing the hillslope-channel contributions to the catchment sediment balance under land use and climate change
9/20
We implemented a morphodynamics module that determines erosion and sedimentation in the rills and channels. The morphodynamics module makes use of a transport capacity algorithm, which is a function of sediment storage and the transport capacity. The morphodynamics module also accounts for trapping of sediment in check dams or large dams.
Slide 10 of Assessing the hillslope-channel contributions to the catchment sediment balance under land use and climate change
10/20
We applied the model to a Mediterranean catchment in southeast Spain. The Taibilla catchment is typical Mediterranean catchment of around 300 km2, with a annual precipitation sum of 500 mm. The main concern regarding sediment yield is the reservoir at the catchment outlet, which is an important source for drink water in the region of Murcia.
Slide 11 of Assessing the hillslope-channel contributions to the catchment sediment balance under land use and climate change
11/20
The land use mainly consists of forest and shrubland. I want to highlight here the Rogativa subcatchment, where my group has been doing a lot of research over the last 20 years, especially related to land use change and the 58 check dams constructed at the end of the 1970s.
Slide 12 of Assessing the hillslope-channel contributions to the catchment sediment balance under land use and climate change
12/20
We calibrated the morphodynamics module using the sediment yield data obtained from the check dams. We used 17 non-silted check dams on the hillslope, which are shown on the left. We did not get very good results for the check dams on the hillslopes, but better results for the ones located in the channels, which were all silted, unfortunately.
Slide 13 of Assessing the hillslope-channel contributions to the catchment sediment balance under land use and climate change
13/20
In the next slide I will show the model results of a land use change scenario we performed in the smaller Rogativa subcatchment. Land use change between 1956 and 2016 is characterized by reforestation, but also by check dam construction, which were constructed at the end of the 1970s to reduce sedimentation of the Taibilla reservoir.
Slide 14 of Assessing the hillslope-channel contributions to the catchment sediment balance under land use and climate change
14/20
The first map shows the amount of sheet erosion, which is the erosion determined by the MMF soil erosion model. The high erosion rates coincide with agricultural land use. The second map shows the erosion and deposition in the rills and channels, which shows that much of the sheet erosion is deposited in the rills, while erosion occurs mainly in the channels.
Slide 15 of Assessing the hillslope-channel contributions to the catchment sediment balance under land use and climate change
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This behaviour is highlighted by this diagram, which shows all the sources and sinks of the sediment balance in the catchment. So most sediment originates from the hillslopes as sheet erosion, but is also largely deposited on the same hillslopes in the rills. The channels contribute more than 35% to the total sediment yield at the catchment outlet.
Slide 16 of Assessing the hillslope-channel contributions to the catchment sediment balance under land use and climate change
16/20
These maps and diagram show the results of the 2016 land use with check dams implemented. Reforestation is responsible for the decrease in sheet erosion. Concentrated flow processes increase slightly (rill and channel erosion), which may be related to a decrease of sediment input from the hillslopes. Ultimately, the check dams capture most sediments, causing a reduction of sediment yield of 82%.
Slide 17 of Assessing the hillslope-channel contributions to the catchment sediment balance under land use and climate change
17/20
We applied the SPHY-MMF model to climate change scenarios in the larger Taibilla catchment. This table shows the climate characteristics of the climate change scenarios, where the 1.5 degree scenario is characterized by an increase in extreme precipitation and the 3 degree scenario by a decrease in precipitation sum.
Slide 18 of Assessing the hillslope-channel contributions to the catchment sediment balance under land use and climate change
18/20
Here I show again the erosion and deposition maps, with most sediment eroded as sheet erosion, but mostly deposited in the rills, as shown in the diagram. At the catchment-scale, concentrated flow processes in the rills and channels contribute most to the sediment yield at the catchment outlet.
Slide 19 of Assessing the hillslope-channel contributions to the catchment sediment balance under land use and climate change
19/20
Under the 1.5 degree climate change scenario we see an increase in all erosion processes, mostly due to the increase in extreme precipitation, leading to an increase in sediment yield at the outlet. Under the 3 degree scenario the sediment yield is projected to decrease with 16%, which may be related to a decrease in annual precipitation.
Slide 20 of Assessing the hillslope-channel contributions to the catchment sediment balance under land use and climate change
20/20
We updated the SPHY-MMF model with channel processes, including an update of the water routing and the implementation of a morphodynamics module. In a model application in a Mediterranean catchment, we show that channel erosion contributes to about 35-40% of the sediment yield at the outlet.