the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Predicting soil moisture across a heterogeneous boreal catchment using terrain indices
- Department of Forest Ecology and Management, Swedish University of Agricultural Sciences, Umeå, 901 83, Sweden
- Department of Forest Ecology and Management, Swedish University of Agricultural Sciences, Umeå, 901 83, Sweden
Abstract. Soil moisture has important implications for drought and flooding forecasting, forest fire prediction and water supply management. However, mapping soil moisture has remained a scientific challenge due to forest canopy cover and small-scale variations in soil moisture conditions. Digital terrain analysis has been suggested as a way forward to model soil moisture variability across landscapes, and multiple digital terrain indices have been developed. However, the performance of these terrain indices depends on the resolution of the digital elevation models used and, in many cases, user-defined index-specific thresholds. In this study, we compared soil moisture predictions using nine different terrain indices and available soil wetness maps, at varying resolutions and user-defined thresholds, with a field dataset of soil moisture registered in five classes from a forest survey covering a boreal landscape. We found that topography could explain the spatial variation in soil moisture conditions but the effects of changing DEM resolution and user-defined thresholds severely affected the performance of the soil moisture modelling. These results demonstrate that modelled soil moisture conditions need to be validated, as selecting unsuitable DEM resolution or user-defined threshold can give ambiguous and even incorrect results. Challenges caused by heterogeneous soil types within the study area highlight the need for local knowledge when interpreting the modelled results.
Johannes Larson et al.
Status: open (extended)
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RC1: 'Comment on hess-2021-560', Anonymous Referee #1, 17 Feb 2022
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The paper by Larson and colleagues deals with the important aspect of predicting soil moisture classes in a study catchment in Sweden. In general, the paper is well written and properly structured and the methods that the authors used to derive various terrain indices are sound. Also worth commending is the large number of field observations n = 398 which were used in the statistical analysis.
A concern for me, however, is the use of the term ‘soil moisture’ throughout the manuscript. Soil moisture and soil moisture classes are not the same thing, in my opinion. Soil moisture is a soil property with both a spatial (lateral and vertical) and a temporal dimension. The soil moisture classes that the authors used (described in line 105 – 133) have only a spatial (and only lateral) dimension. It is therefore rather a mapping unit or a soil association than a soil property. The qualitative description of the soil classes also appears to be biased e.g., possible to walk over and keep dry feet…shortly after snowmelt. Surely this will depend on the size of the person and how long ‘shortly’ is. Also, the topographic descriptions of the soil moisture classes are raising questions about potential circular reasoning. How important is these topographic attributes in determining the soil moisture class? If they are a key determining factor, then surely you are not assessing whether the terrain indices are predicting soil moisture, but rather are the terrain indices able to predict terrain indices. So, in my opinion, the authors did not predict soil moisture, making the title and a lot of the discussion misleading.
With this being said, the prediction of soil moisture classes (soil associations) is still novel, and the paper makes a contribution to international literature. An important outcome is that a finer detailed DEM does not necessary imply better predictions of soil moisture classes.
The article would benefit by adding an improved description of the climate under section 2.1
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CC1: 'Reply on RC1', Jose Gutierrez Lopez, 23 Feb 2022
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Interesting observations.
One detail that I believe should be added, is that "soil moisture" is just an umbrella term that might include quantifiable, and unquantifiable amounts of water. The spatiotemporal and quantifiable soil moisture can be "volumetric water content" or "gravimetric water content", which once normalized for porosity gives us some form of saturation (or extractable water, as some authors in ecology call it)
Yes, I agree, and I believe authors predicted "soil wetness", or "aparent soil saturation", but neither soil moisture, nor the ones I suggest are quantifiable. So the use of "soil moisture", is not necesarily inadequate.
Perhaps authors can make this distinction in the introduction and methods, if they feel it is necesary within their field of study
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CC1: 'Reply on RC1', Jose Gutierrez Lopez, 23 Feb 2022
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Johannes Larson et al.
Johannes Larson et al.
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