In #1-6 we looked at trends in Tropical Cyclones. In #7-#9 we looked at trends in extreme rainfall and floods. Now we move onto droughts. The simplest idea about droughts is they occur when there’s a drop in rainfall over some time period. Rainfall is relatively easy to measure. However, as a caveat, the world is a big place and different datasets have differences.
If you are interested, here are some basics that I'd like to understand better.
1) I believe, but don't fully understand, that drought is always quantified relative to some normal condition and measured in terms of standard deviations from that norm. 1 m of rain in the Amazon could be called a drought while 20 cm in a desert may be a wet year. If so, is a certain percentage of the planet always expected to be in drought or severe drought all of the time? In other words, how much droughts are simply natural variability?
2) If I understand correctly, since most land surfaces are covered with vegetation, most evaporation is technically "transevaporation" mediated by plants. Evaporation of liquid water is expected to increase at 7%/degC at constant windspeed. However, plants can close their stomata and greatly reduce their need for water. Is transevaporation expected to increase at 7%/degC at constant wind.
3) The dramatic reduction in lakes behind dams on the Colorado River is THE hot topic in drought in the US at the moment. I spent some time trying to determine whether this has occurred because of a decrease in rainfall (there was less in the 2000s), about 1? degC of warming in the watershed driving evaporation, or simply more use of water for farms, lawns, golf courses and even pools (mandated pool covers to cut evaporation)? For example, Denver pumps water that would normally flow into the Colorado over the continental divide from a reservoir on the western side near where I was skiing this winter. Could these massive reservoirs on the Colorado that took a decade or more to fill really be so low simply because we are using too much water?
If you are interested, here are some basics that I'd like to understand better.
1) I believe, but don't fully understand, that drought is always quantified relative to some normal condition and measured in terms of standard deviations from that norm. 1 m of rain in the Amazon could be called a drought while 20 cm in a desert may be a wet year. If so, is a certain percentage of the planet always expected to be in drought or severe drought all of the time? In other words, how much droughts are simply natural variability?
2) If I understand correctly, since most land surfaces are covered with vegetation, most evaporation is technically "transevaporation" mediated by plants. Evaporation of liquid water is expected to increase at 7%/degC at constant windspeed. However, plants can close their stomata and greatly reduce their need for water. Is transevaporation expected to increase at 7%/degC at constant wind.
3) The dramatic reduction in lakes behind dams on the Colorado River is THE hot topic in drought in the US at the moment. I spent some time trying to determine whether this has occurred because of a decrease in rainfall (there was less in the 2000s), about 1? degC of warming in the watershed driving evaporation, or simply more use of water for farms, lawns, golf courses and even pools (mandated pool covers to cut evaporation)? For example, Denver pumps water that would normally flow into the Colorado over the continental divide from a reservoir on the western side near where I was skiing this winter. Could these massive reservoirs on the Colorado that took a decade or more to fill really be so low simply because we are using too much water?