23 Apr 2012

Making Revision 'Fun' Part 1: Geography and the Drought

Okay, I've been debating whether or not to use my blog as a place to post stuff related to my GCSEs (provided I find it remotely interesting), and this Guardian article tipped me over the edge into "sure why not". Note that, since this is a revision post, the bolding is mostly for my benefit, and (aside from highlighting the note at the end) is just the keywords.

As you might have guessed by the URL at the very least, it's about how the recent bout of heavy rain and why, although it is capable of causing flash floods in some areas of the country, it isn't magically going to undo the drought we've been in for nearly 2 years now. I've decided to dedicate this post, then, to writing about the stuff I need to know about rain for my Geography course. Any misleadingness in this is all my fault though (this doesn't need saying, but I'm a GCSE student, not a meteorologist.).

First of all, theres 3 main 'types' of rain formation: convectional, relief and frontal (or, to use their Wikipedia Names, convection, orographic and stratiform).

Relief/orographic rainfall generally occurs on the side of mountains opposite to the prevailing wind (making a lot of the drought/ hit areas in the 'rain shadow' (where there's less rainfall)) and is relatively constant compared with the other types. Obviously, this is probable where there's mountains.

Convectional is the stuff which causes thunderstorms: when there's enough of a difference in temperature witha moist/unstable atmosphere for a whole ton of water to evaporate (well, several tons of water, in all likelyhood), form clouds, and fall to the ground as rain hard and fast (all that energy has to go somewhere, and warmer air can 'hold' more water). The Wikipedia's description is more accurate that the GCSE stuff obviously - for it we're mainly shown the model of it happening in tropical rainforests, but judging by the presence of hail in some of the rain we've had over here, it looks like we're getting a bit of convectional rainfall at least (won't go as far as to say how much or anything like that; not a meteorologist (I can't even spell "meteorologist" without the help of a spell checker)).

Finally, frontal/stratiform rainfall is the stuff we associate with the black lines that occassionally appear on the weather map - weather fronts - you can probably see an example here. There's an entire page on the Wikipedia dedicated to them, if you're interested. The lines with the blue triangles on are the cold fronts, the red semicircles the warm fronts and where there's a mix of both occluded fronts, or possibly stationary fronts according to the Wikipedia (a purple mix being an occluded front, although I'm fairly certain the BBC uses the first notation I mentioned).  Behind the cold front it's colder, behind the warm front it's warmer ("warm sector"), and the occluded front depends on whether it's caused by the warm overtaking the cold, or vice versa. The fronts themselves cause rain as the difference in temperature means the less dense (but possibly a bit damp) warmer air rises, the water vapour contained within condenses into clouds, and then there's rain. I'll be willing to bet there's a bit of frontal rainfall in the mix.

Types of rain and wanton weather speculation aside, the main issue is that the ground water stocks are being depleted and river levels will remain low once the run-off runs out. This is in part caused by urbanisation, but it's aggravated by the fact that the ground was dry from the drought (making it less absorbent). The reason that we can be in drought and have flood warnings is that there's a ton of run-off which results from this (plus the fact that the rain was pretty heavy and antecedent; it was falling where the ground was saturated, so it could only run-off into the rivers).

These issues are in part due to stuff in the water cycle. The idea is, water hits the ground (some being intercepted by trees), some runs off, but some infiltrates the soil. Eventually, the soil becomes saturated, and (if the rock below it is permeable (let's say limestone for now)) some of the water percolates through the rock, replenishing the groundwater flow and raising the water table (which is low ATM due to the lack of rain). At the same time, some may run through the soil (a process which I swear is slightly different to groundwater flow, but which I can't find the name for).

In addition to this, plants do 'suck up' (for want of a less inaccurate word) water and release it out/transpirate it (for evaporation) which also obviously reduces the amount of water in the soil but reduces surface run off (trees also help to stop the soil washing away and 'intercept' the water, making it reach the ground less quickly).

This does mean that, aside from the obvious issue of climate change, human action has had another (but obviously not total) hand in this present situation: urban areas (which pretty much have to be in a river's catchment area, and there's now also the issue of building in the flood plain) are paved, and necessitate the cutting down of trees. Deforestation obviously stops trees from doing the tree things (/Buffy speak) mentioned before, but the paving of areas in a drainage basin presents a two-fold problem aside from this: water can't intercept waterproof concrete, so it runs off into rivers at a faster rate, which helps to cause flooding (in combination with unusual amounts of rainfall within the drainage basin). In addition to this, it's impossible for water to percolate down and replenish the groundwater stocks. I'm guessing the main reason why this is only becoming a mega problem now is that groundwater flow is slow, it can take weeks, months, years (millenia in some cases) for it to reach the river, so whilst this means a steadier supply of water, the effects might only have become an issue more recently than the start of the drought.

Note on the definition of "drought": There are about a number of different definitions used to define drought in the UK, even just sticking with meteorological droughts. The first, within the shortest term and the one I was taught, is at least 15 consecutive days with less than 0.2mm of rain (in total), whereas longer term definitions are 3 months with a "50% deficit" in rainfall or 2 years with a 15% shortfall.[1] This drought is in one of the latter categories, probably (due to the reference in the Graun article to the rainfall over next winter determining if we stay in drought) the last one.

There's also hydrological drought and agricultural drought.

[1] Phillip Eden: Water Shortage Coming?-Weather UK (2007, link retrieved April 2012).

No comments:

Post a Comment