Tag Archives: hydrology

maybe global warming isn’t causing as much drying as we thought?

I certainly have had the impression that both agriculture and natural ecosystems are becoming more water scarce due to global warming, and that this is going to be a big problem at some point. We hear that the Amazon (River basin) may be tipping into an arid ecosystem, which has implications for the entire global climate and food supply, for example. But this article in Water Resources Research suggests there may be some feedback loops being overlooked. If I can try to summarize in a couple sentences, the concern is that higher temperatures cause greater evaporation from soil and transpiration (evaporation of soil moisture through the pathway of plant roots and leaves), and this will lead to drying both in agricultural and natural ecosystems. But plants have mechanisms to resist this loss of moisture, specifically by closing stomata which are the openings in leaves through which transpiration takes place. The mechanism can offset some but not all of the increased drying effect.

The CO2 Balancing Act: Why Global Warming and Greening Don’t Dry Earth as Much as We Thought

While air warming and vegetation greening are widely assumed to intensify terrestrial drying through enhanced evapotranspiration, rising atmospheric CO2 concentration ([CO2]) may counteract this effect by inducing stomatal closure and reducing water depletion. However, the complex interplay between these factors has obscured their net impact on global terrestrial drying. Here, we develop a model that physically and effectively quantifies the relationships among evapotranspiration, [CO2], and climate and vegetation changes, which can explicitly reflect how CO2-mediated stomatal regulation interacts with climate and vegetation changes to modulate evapotranspiration. We find that, globally, the drying effects of warming and greening are largely offset by CO2-induced reductions in surface conductance (69.4% ± 16.9%) and associated meteorological feedbacks. This compensatory mechanism is overlooked in traditional drying indicators, that is, the Palmer Drought Severity Index (PDSI) overestimates trends in drought-affected area (63.2% ± 10.1%), drought duration (58.7% ± 9.5%), and drought intensity (43.9% ± 7.7%) during 1982–2014 by ignoring CO2-vegetation-climate interactions, and similarly, potential evapotranspiration-based aridity index underestimates wetting trends by 66.1% ± 3.5%. Our results reveal a systematic bias in current global drying assessments, which exaggerate drying in aridifying regions while underestimating wetting trends elsewhere. These findings reinterpret the hydrological impacts of global change, demonstrating that [CO2] rise acts as a critical buffer against terrestrial drying. The study provides a mechanistic framework to reconcile observed greening with hydrological trends, offering transformative insights for ecohydrological modeling and water resource management in a high-[CO2] climate.

climate change and Hurricane Harvey

Michael Mann, a climate scientist at Penn State, has posted a long (for Facebook) article on Facebook about how climate change contributes to events like this. In short, climate determines the probability of a particular weather event occurring, but ultimately any one particular weather event is a roll of the (now slightly loaded) dice. Warmer water and warmer air than in the past have both made events like this more likely, and are making events like this more destructive when they do occur. The article has links to several journal articles which would be worth reading to know something about hydrology and climate change. But right now I can’t do that because I’m late for my job where I have to convince people I know something about, among other topics, hydrology and climate change.

Sea level rise attributable to climate change (some is due to coastal subsidence due to human disturbance e.g. oil drilling) is more than half a foot over the past few decades (see http://www.insurancejournal.com/…/sou…/2017/05/31/452704.htm for a decent discussion).

That means that the storm surge was a half foot higher than it would have been just decades ago, meaning far more flooding and destruction.

In addition to that, sea surface temperatures in the region have risen about 0.5C (close to 1F) over the past few decades, from roughly 30C (86F) to 30.5C (87F), which contributed to the very warm sea surface temperatures (30.5-31 C or 87-88F). There is a simple thermodynamic relationship known as the “Clausius-Clapeyron equation (see e.g. https://en.wikipedia.org/…/Clausius%E2%80%93Clapeyron_relat…) that tells us there is a roughly 3% increase in average atmospheric moisture content for each 0.5C (~1F) of warming. Sea surface temperatures in the area where Harvey intensified were 0.5-1C warmer than current-day average temperatures, which translates to 1-1.5C warmer than the ‘average’ temperatures a few decades ago. That means 3-5% more moisture in the atmosphere.

launching the cosmic-ray neutron probe…


Okay this isn’t really about an evil plot to dominate the world. It’s about measuring soil moisture.

Estimating field-scale root zone soil moisture using the cosmic-ray neutron probe

Many practical hydrological, meteorological, and agricultural management problems require estimates of soil moisture with an areal footprint equivalent to field scale, integrated over the entire root zone. The cosmic-ray neutron probe is a promising instrument to provide field-scale areal coverage, but these observations are shallow and require depth-scaling in order to be considered representative of the entire root zone. A study to identify appropriate depth-scaling techniques was conducted at a grazing pasture site in central Saskatchewan, Canada over a 2-year period. Area-averaged soil moisture was assessed using a cosmic-ray neutron probe. Root zone soil moisture was measured at 21 locations within the 500 m  ×  500 m study area, using a down-hole neutron probe. The cosmic-ray neutron probe was found to provide accurate estimates of field-scale surface soil moisture, but measurements represented less than 40 % of the seasonal change in root zone storage due to its shallow measurement depth. The root zone estimation methods evaluated were: (a) the coupling of the cosmic-ray neutron probe with a time-stable neutron probe monitoring location, (b) coupling the cosmic-ray neutron probe with a representative landscape unit monitoring approach, and (c) convolution of the cosmic-ray neutron probe measurements with the exponential filter. The time stability method provided the best estimate of root zone soil moisture (RMSE  =  0.005 cm3 cm−3), followed by the exponential filter (RMSE  =  0.014 cm3 cm−3). The landscape unit approach, which required no calibration, had a negative bias but estimated the cumulative change in storage reasonably. The feasibility of applying these methods to field sites without existing instrumentation is discussed. Based upon its observed performance and its minimal data requirements, it is concluded that the exponential filter method has the most potential for estimating root zone soil moisture from cosmic-ray neutron probe data.

what is a blizzard?

According to Five Thirty Eight,

Three factors are required for a storm to be classified as a blizzard at a particular place, besides falling or blowing snow:

1. Sustained winds or frequent wind gusts of 35 mph or greater

2. Visibility under a quarter-mile

3. These conditions must persist for three hours.

This definition is the same whether you’ve got 1 inch or 40 on the ground.