Tag Archives: hydrologic cycle

evaporation energy

There is a lot of energy in evaporation, and there are technologies that theoretically could harvest it for human use.

About 50% of the solar energy absorbed at the Earth’s surface drives evaporation, fueling the water cycle that affects various renewable energy resources, such as wind and hydropower. Recent advances demonstrate our nascent ability to convert evaporation energy into work, yet there is little understanding about the potential of this resource. Here we study the energy available from natural evaporation to predict the potential of this ubiquitous resource. We find that natural evaporation from open water surfaces could provide power densities comparable to current wind and solar technologies while cutting evaporative water losses by nearly half. We estimate up to 325 GW of power is potentially available in the United States. Strikingly, water’s large heat capacity is sufficient to control power output by storing excess energy when demand is low, thus reducing intermittency and improving reliability. Our findings motivate the improvement of materials and devices that convert energy from evaporation.

This is interesting. Cutting evaporation losses in half could be a good thing in some situations, like reservoirs and swimming pools in arid regions. Cut too much evaporation elsewhere, and you could imagine a science fiction scenario where you have a full reservoir but nearby ecosystems or farmland turn into deserts. Or you end up pumping that reservoir and using it for irrigation using the energy you have harvested, in the end using technology to efficiently recreate the hydrologic cycle and ecosystem services nature used to provide for free.

the urban cool island

Here’s an article on quantifying the urban cool island. Which, as you might expect, is the opposite of the urban heat island.

Quantifying the cool island effects of urban green spaces using remote sensing Data

Urban Heat Island (UHI) leads to increased energy consumption, aggravated pollution and threatened health of citizens. Urban green spaces mitigate UHI effects, however, it is still unclear how the green space characteristics and its surrounding environment affects the green space cool island (GCI). In this study, land surface temperature (LST) and land cover types within the outmost ring road of Shanghai, China were obtained from Landsat 8 data and high-resolution Google Earth data. The GCI effects were defined in three aspects: GCI range (GR), amplitude of temperature drop (TA) and temperature gradient (TG). Pearson correlation analysis was processed to get the relationship between the aspects and impact factors. The results indicated that the GCI principle could be explained by the thermal conduct theory. The efficient methods to decrease LST of green spaces include increasing green space area while staying below the threshold, adding complexity of green space shape, decreasing impervious surfaces and enlarging the area of water bodies. For the surrounding environment of the green spaces, increasing vegetation and water body fractions or decreasing impervious surfaces will help to strengthen GCI effects. The findings can help urban planners to understand GCI formation and design cool green spaces to mitigate UHI effects.

This is a subject where I’m out of my depth in terms of formal training, but certainly interested. There are at least two ways you can try to combat the urban heat island effect, which occurs when pavement and other man-made surfaces absorb heat during the day and release it slowly at night (and during the day). The first is to use light-colored materials to reflect sunlight back into space. Using white roof materials whenever practical seems like a no-brainer. Maybe we don’t want snow white paving materials everywhere at the ground level, because that could be displeasing and even painful to the eye, but certainly we could dispense with the asphalt. Even if asphalt didn’t absorb heat, it would still be a hideous, toxic, short-lived material. It’s better to use concrete or brick or stone or almost anything else – it may cost more up front but it will last longer and just generally make our urban areas better. Materials that are permeable to rain water are also available so let’s consider those where they make sense.

The other way is to maximize the use of soil and vegetated surfaces. Soil and vegetated surfaces also absorb heat, I think, but then dissipate much of it again through evaporation and transpiration. Then there is the simple process of tree canopy create shade at ground level (which I imagine satellite studies like the one above may have trouble picking up on). In very dry climates, this may not be practical because to state the obvious, you need water to have evaporation. In very, very wet climates, it might make sense to store rainwater and intentionally spray it on your paved surfaces to cool them down. This is assuming you want to get rid of the heat and water – if you are in a place where water is scarce and precious, you might not want to do that, and you might even want to think twice about having a lot of vegetated surface. Or maybe that is not the right place for large numbers of people to live. Unless you can create more or less a closed-loop water system, in which case it might be a good place, thinking in terms of ecological footprint and preparing for humanity’s possible future in space.

residental graywater

Here’s an interesting report on the economics of residential graywater systems. It’s a little wishy-washy (no pun intended, ha) on the numbers, but it has some links and references that could be useful. From a quick skim, it suggests that if you can achieve a savings of about $200 a year, your system will break-even over a typical service life of around 15 years. This is more likely to happen if you have a relatively high number of people in your house and if you have relatively high water rates.

I have a low-tech, essentially free graywater setup. I turn on the shower, wash my hair and face with weird chemicals, stop up the drain, and wash the rest of myself with pure, non-toxic biodegradable soap. Then, I use a bucket to collect water for houseplants and outdoor plants. I check with NOAA online to see if there has been less than an inch of rain over the past 7 days (a very rough rule of thumb for evapotranspiration around here) and to see if there is rain expected over the next day or so. If I’m diligent about this in the summer, I end up not having to get out the hose too often. Combine all this with a rain barrel or two and I would have to get the hose out even less often.

If I were to accidentally pee in the shower…well, I’ll take the 5th on that one but I’m pretty sure the plants wouldn’t mind.

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.

green roofs

Here’s a green roof modeling study from Singapore. Green roofs reduce peak flows enough to help with flooding. They reduce the volume of runoff a little bit through increased evapotranspiration, which would have an effect on the water supply in Singapore where urban runoff is used as a water source.

Effect of Catchment-Scale Green Roof Deployment on Stormwater Generation and Reuse in a Tropical City

Low-impact development (LID) comprises a broad spectrum of stormwater management technologies for mitigating the impacts of urbanization on hydrological processes. Among these technologies, green roofs are one of the most adopted solutions, especially in densely populated metropolitan areas, where roofs take up a significant portion of the impervious surfaces and land areas are scarce. While the in situ hydrological performance of green roofs—i.e., reduction of runoff volume and peak discharge—is well addressed in literature, less is known about their impact on stormwater management and reuse activities at a catchment or city scale. This study developed an integrated urban water cycle model (IUWCM) to quantitatively assess the effect of uniform green roof deployment (i.e., 25, 50, and 100% conversion of traditional roofs) over the period 2009–2011 in the Marina Reservoir catchment, a 100-km2100-km2, highly urbanized area located in the heart of Singapore. The IUWCM consists of two components: (1) a physically based model for extensive green roofs integrated within a one-dimensional numerical hydrological-hydraulic catchment model linked with (2) an optimization-based model describing the operation of the downstream, stormwater-fed reservoir. The event-based hydrological performance of green roofs varied significantly throughout the simulation period with a median of about 5% and 12% for the catchment scale reduction of runoff volume and peak discharge (100% conversion of traditional roofs). The high variability and lower performance (with respect to temperate climates) are strongly related to the tropical weather and climatic conditions—e.g., antecedent dry weather period and maximum rainfall intensity. Average annual volume reductions were 0.6, 1.2, and 2.4% for the 25, 50, and 100% green roof scenarios, respectively. The reduction of the stormwater generated at the catchment level through green roof implementation had a positive impact on flood protection along Marina Reservoir shores and the energy costs encountered when operating the reservoir. Vice versa, the drinking water supply, which depends on the amount of available stormwater, decreased due to the evapotranspiration losses from green roofs. Better performance in terms of stormwater reuse could only be obtained by increasing the time of concentration of the catchment. This may be achieved through the combination of green roofs with other LID structures.

An R package for modelling actual, potential and reference evapotranspiration

I and some people I know will be very excited about this. Which means I know some very weird people.

An R package for modelling actual, potential and reference evapotranspiration

Evapotranspiration (ET) is a vital component of the hydrological cycle and there are a large number of alternative models for representing ET processes. However, implementing ET models in a consistent manner is difficult due to the significant diversity in process representations, assumptions, nomenclature, terminology, units and data requirements. An R package is therefore introduced to estimate actual, potential and reference ET using 17 well-known models. Data input is flexible, and customized data checking and pre-processing methods are provided. Results are presented as summary text and plots. Comparisons of alternative ET estimates can be visualized for multiple models, and alternative input data sets. The ET estimates also can be exported for further analysis, and used as input to rainfall-runoff models.