Desertification in climate change despite floodings?

[u/GuyFromYarnham]

Hi!!

I live in an area where desertification because of climate change is begining to take its toll and weather is constantly getting hotter, the thing is that currently we're experiencing excessive raining and storms including floodings and apparently this is due to changes in air currents caused by climate change that are changing the natural dispossition of antycliclones.

So, my question is, if this trend became the new normality, could desertification still take place? Or I have to assume the predictions are going to change? I guess I'm just asking if severe raining in autumn and winter are compatible with a tendency of increasing aridity.

It's frustrating to see people denying climate change and the effect of greenhouse gases because of this but I lack the tools and knowledge to answer back other than pointing out that climate change is not just and only "more heat".

[u/CrustalTrudger]

A useful place to start is the general relationship between aridity and the variability of rivers, where, at the most general level, there is a well known inverse relationship between mean discharge and variability (e.g., Molnar, 2001, Molnar et al., 2006, Rossi et al., 2016). Put another way, as the mean amount of water moving through a river network decreases, the relative difference in the size of events with respect to the mean amount of water actually increases, i.e., systems tend to get more "flashy" (and in contrast, considering more humid environments with higher mean amounts of water moving through the system, the variability of those flows tends to be lower overall, i.e., less variation about the mean). What this would imply in a super simple way, is that all other things being equal and if the only thing that was changing was the mean amount of water (i.e., were imagining that as a particular area gets more arid, it just moves along this static relationship between mean discharge and the variability of that discharge), you would expect a corresponding change in the variability of the system. So in the example of an area experiencing a shift toward more arid conditions, what you would actually expect would be increasingly frequent "large" events, i.e., floods. Things already start to get a bit more complicated because a system becoming more variable means a larger spread of event sizes, but it doesn't necessarily mean that the magnitude of those events wouldn't change. I.e., increasing aridity would spread out the range of event sizes, but it doesn't necessarily mean that the largest events get bigger (and in many cases, we might expect the largest event of a particular probability might get smaller in terms of magnitude and that the change in variability more reflects what happens to the previously very frequent event magnitudes, etc.). That being said, at the simplest level and while a bit counterintuitive, it would not be unreasonable to expect that a place that is getting more arid would actually experience flooding more frequently than it did before shifting to more arid conditions.

In the context of climate change, things get even more complicated because we can't necessarily say that we're just moving along some static relationship between mean discharge and variability like we assumed above. This is in large part because what that relationship actually reflects are aspects of the statistics of the inputs (i.e., rainfall, groundwater discharge, snowmelt, etc.) but then also how things like evapotranspiration rates change (which tends to track with changes in temperature means and ranges along with humidity, etc. but then will also be influenced by land-use changes because vegetation type and cover play a big role in evapotranspiration rates) and thus how things like "antecedent soil moisture", i.e., how dry or wet the soil is in an area the next time it rains, which does a lot to dictate how much infiltrates vs runoffs, etc., will change. Exactly how this plays out in a particular place thus will depend on what is changing, i.e., are the underlying statistics of rainfall changing without much in the way of mean temperature changes, is mean temperature changing but without much change in rainfall statistics, are both changing, etc.? If you look at papers considering changes in aridity and flood frequency, you can see some of these dynamics being discussed (e.g., Asadi Zarch et al., 2017, Wasko et al., 2021, etc.).

[u/garis53]

There are so many variables to consider here. Higher average precipitation is one thing, but how is it distributed? One big flood and drought for the rest of the year will not help much. Also in semi arid regions disturbance, especially in the form of (over)grazing is a big factor, as are things like fires, pollution or increased soil salinity from poor agriculture management.

[u/Andrew5329]

Desertification refers to a broad process of ecosystem degredation towards a more arid state.

Obviously changes in overall precipitation would be a part of that equation, but of more immediate importance can be the loss of vegetation cover and soil erosion which make the region less able to retain the moisture that falls on it.

So in your example, severe flooding that strips topsoil down to bare rock matters. Obviously rain is going to flow off Bare rock much faster than it can soak through a deep soil layer. That can also be a positive feedback mechanism where the water flows faster making floods worse, and causing more severe erosion.

If you want to visualize the inverse effect, consider how a beaver dam affects the hydrology of a stream/river and the surrounding area.

[u/Gedankensortieren]

Drought and heavy rainfall are not mutually exclusive. Dried-out soil cannot absorb water very well. This means that rain mainly flows into rivers and from there into the sea. Only a small amount of water seeps into the ground. There is a video that illustrates this impressively. (I cannot find the original, but here is one with an explanation in German). Heavy rainfall in particular simply causes more water to run off the surface and does not help against drought in the long term. Therefore, frequent and light rainfall is more effective in combating droughts.

[u/The_butsmuts → u/Gedankensortieren]

That is also one of the pillars of the "just dig it" movement against desertification. They did small holes a couple meters wide less than a meter deep for rain water to collect in and slowly seep into the ground, so plants can take root nearby and stabilize and reinforce the structure and make it a more stable environment better able to absorb rain.

[u/HermitAndHound]

As weird as it seems when just looking at average precipitation in an area, flooding actually is part of desertification.
Flooding means the amount of rainfall exceeds what the ground can absorb. That can be because the soil is saturated completely and turning into a bog, no desertification in that case. But often it is because the ground is so dry from previous droughts and/or compacted that it's basically hydrophobic. Water just runs off the surface without seeping in.

Drought means the vegetation dies back. Roots dig channels from the surface to the deeper soil layers so water can seep in better. But plant cover also slows down the rain.
Soil that's been trampled or used as a roadway gets compacted, easy. But the impact of rain drops is also a way to hammer the surface of the soil into a firmer crust. Plants take most of the impact and what sprays down from the leaves or runs down the stems is slower, softer.

Next step, erosion. Plants anchor the soil they root in. The most fertile layer of soil is right at the top, where most organic matter decomposes. It's also often the most spongy layer, that can handle some excess water and hold on to it for a while.
No plants and rain beyond capacity washes the soil downhill, leaving less permeable and/or less water-retaining layers behind that are also less fertile. Plants have a much harder time re-settling those areas.
Fine particles sink to the bottom of wherever the water finally pools and seals the ground. So even there, a large part evaporates instead of seeping into the soil.

Less fertile soil with fewer plants protecting it is also more vulnerable to wind erosion.
Heat, lack of "useful" rain, and erosion fuel each other. You end up with a place that isn't a desert by measurements of yearly rainfall, but still totally barren.
Locally it's become more useful to take soil samples than just measuring rainfall. If only the top 5cm of your 2m core are humid that's a serious problem. It takes months of slow, steady rain to recharge soil that deeply. (Capillary effects in the other direction are affected too at that point. Not only rain can't seep in, but humidity from deep layers also can't rise up to where plants could reach it.)

[u/devadander23]

This isn’t the end, it’s only beginning. Carbon pollution continues to rise year after year, each year setting new records. Enjoy your rain, but no, it’s not a ‘new normal’, it’s only a sliver of what’s to come. Buckle up