The Induced Precipitation Recycling (IPR) concept can be summarized as an approach to increase regional precipitation, by capitalizing on the natural interactions between forest cover and weather patterns (also known as climate-vegetation feedbacks). These feedbacks can be initiated in an arid region by reforesting marginalized land, using wastewater for irrigation.  As moisture is transpired by the trees, it becomes available to fall as rain. Increased rainfall helps establish more forest cover, which increases moisture transpiration.  As forest cover increases and progresses inland, other factors come into play that increase precipitation yields well beyond what was supplied through irrigation. Ultimately, the environment is favorably modified to continuously strengthen these effects.

Planting trees is not usually considered a way to increase the water supply. In fact, conventional wisdom states that trees only consume water. Traditional research has demonstrated that reducing forest cover increases localized storm run-off, which is then added to the water supply. However, a growing body of scientific research indicates that increasing forest cover can actually promote precipitation across a larger region and intensify the water cycle, thereby contributing much more to the available water supply.

Therefore, the role of forests and their impact on precipitation, water yield and the hydrologic cycle is contested. The scientific debate can be divided into two schools of thought (click here for more detail):

• Demand Side:  views trees and forests as consumers of available water, and competitors for other downstream water uses

• Supply Side:  contends that increasing forest cover raises overall water yield

The IPR concept is based on research supporting the Supply Side perspective. New insights into the role of forests and their impact on regional precipitation, water yield (how they affect the local water supply) and the hydrologic cycle have been derived from the results. Our methodology was developed as a practical engineering application based on this research. To understand how IPR can increase rainfall, some background information is helpful. Two natural processes known as precipitation recycling (PR), and inland moisture flow, form the basis for our approach. These processes are summarized below, however much more detail can be found here.

Precipitation Recycling

Forested land, and to a lesser extent agricultural land, recycle rainwater through a combination of evaporation and transpiration (in essence, water released through the leaves of a plant during photosynthesis), collectively known as “evapotranspiration” or ET. So, water that has been consumed by vegetation is released to the atmosphere, becoming available to fall again as rain.  Potentially, the same water can return as rain several times in the same catchment basin or neighboring regions. In fact, this effect can supply anywhere from 25-50% of the annual precipitation in the Amazon region, and as much as 30% in the sub-Saharan region in Africa. While the extent and impact of precipitation recycling varies significantly from region to region, it has been observed in many locations, under a wide variety of conditions. Importantly, it occurs in arid regions (such as the southwestern deserts of the United States) as well. Observations also suggest that extensive deforestation often reduces cloud formation and rainfall, even if the cleared land is used for agriculture. This implies that an important source of atmospheric moisture is lost when forest cover is removed.

Inland moisture flow

The importance of precipitation recycling for creating and maintaining forest cover is more apparent further inland. Most atmospheric moisture that can fall as rain originates as evaporation from the ocean. Most rainfall from this source falls within a couple hundred miles of the coast, decreasing rapidly as the distance increases. The more distant a region is from a coast, the more difficult it becomes to directly transport this  moisture from the ocean. Precipitation recycling provides a way to do this. It is now well known in the scientific community that forests and vegetation cover play an important role in transporting moisture inland by returning water vapor back to the atmosphere through ET.

Coastal forest cover may also contribute in other ways to local weather patterns. The biotic pump theory suggests that forests modify prevailing winds in a manner that can further intensify moisture transport and regional precipitation.  Conversely, inadequate forest and vegetation cover may exacerbate adverse atmospheric conditions in semi-arid and arid regions (as described here). This theory contends that the world’s forests (or lack thereof) strongly influence precipitation patterns in continental interiors. Although the postulated large-scale forest-climate interactions are supported by a growing body of observational evidence, the atmospheric dynamics and accompanying theoretical calculations are still highly contested. However, aspects of this theory appear to provide new insight into how significant precipitation levels may be supplied deep within continental interiors, such as happens in the Amazon and Congo river basins. The basic principles are illustrated in Figure 1.

1. ET from forested regions (and wetlands) generates “moist” air (more precisely, increases relative humidity). “Moist” air is more buoyant than “dry” air and rises. 

2. As the air rises and cools, water vapor condenses to form droplets and clouds.  This process further increases the buoyancy of the rising air, creating a low-pressure region above the forest.

3. Evaporation also occurs over the ocean, but not as much as over forests.

4. When evaporation is stronger over the forest than over the ocean, a lower pressure region exists over the trees.  Moisture-laden air from the ocean is drawn towards the forest, generating wind that helps drive moisture further inland.

5. This effect extends further from the coast as the moisture is recycled in stages, moving further inland. As a result, moisture can be consistently transported thousands of miles into the interior of a continent.

Just how much forest cover is required for a strong biotic pump effect to exist is not well known. The scientists who proposed and developed the theory suggest extensive forest cover on the order of 100’s to 1000’s of square miles may be required. The general hypothesis, however, implies that this effect may occur to some extent whenever sufficient ET is produced in coastal regions and further inland. This in turn influences local wind conditions (in varying degrees depending on the strength of the effect) to increase the flow of moisture inland.

Induced Precipitation Recycling (IPR)

IPR is based on the concept that precipitation recycling can be induced to initiate and strengthen the naturally occurring processes described above (hence IPR). This is accomplished by reforesting (or afforesting, if there hasn’t been forest cover in recent history) marginalized land, using wastewater (or storm runoff) for irrigation. ET generated from these newly vegetated areas directly augments existing atmospheric moisture to further promote and intensify the water cycle.  Any increase in local precipitation results in enhanced runoff, thereby adding to the current water supply.

In addition, any increase in precipitation from this will support additional new vegetation and forest growth.  This implies a distinction between cultivated and secondary growth:

1. Cultivated growth refers to reforested or afforested land which is directly irrigated with waste water or storm runoff, etc.

2. Secondary growth refers to forest growth in a location that receives increased precipitation as a result of environmental modifications and/or increased atmospheric moisture attributable to the IPR project

As the amount of cultivated and secondary forest growth increases from an IPR project site, more water is expected to be recycled through ET. Natural orographic features downwind of the site (hills, mountains, etc.) help increase the probability that a greater share of ET will precipitate locally, as rainfall or condensing fog. This in turn is expected to increase secondary growth, as shown in Figure 2.

As forest cover increases, ET from the trees (and any emerging “biotic pump” effects) will enhance the flow of moist air from the ocean, driving atmospheric moisture further inland. These conditions support a virtuous cycle where increased precipitation encourages more secondary growth (without increased irrigation), and the additional growth generates additional ET to further increase precipitation (Figure 2).  Increased runoff is considered a natural consequence of this virtuous cycle, which benefits the regional water supply.

Importantly, IPR need not require the depletion of existing water resources to function. Water from alternative sources not directly linked to the available water supply (e.g. excess storm runoff, treated sewage, fog capture, etc.) can provide the required irrigation for new vegetation growth without reducing current downstream availability. This is a significant consideration for establishing IPR in semi-arid regions – atmospheric moisture can be increased from an unused source with limited alternative value. In fact, using excess storm runoff and processed sewage has considerable environmental value, since forests can purify this water. The reduction in pollution from these sources represents a significant added bonus. Such potential spinoff or co-benefits raise the overall added-value and represent an important aspect of the proposed IPR implementation model.

Transitioning from Theory to Reality

Because the underlying theories behind IPR haven’t been formally tested yet, preliminary research is necessary to verify the viability of the approach. The desired result of the research is to show direct evidence that IPR can initialize the processes described above, thus validating IPR as a viable approach to increasing regional water supplies. A series of test plans have been developed, as described here.

The eventual success and value of an IPR project is highly dependent on where the project is located. Factors such as physical geography, proximity to the ocean and availability of land for reforesting will strongly influence the outcome. These need to be considered when selecting locations for potential IPR projects.

Finally, the overall effect of the project will also vary depending on conditions at the project site and the surrounding region. A typical IPR project would be considered successful if it increased the local water supply by a measurable amount, for an acceptable cost (these terms are of course subjective). On a larger scale, we believe that the processes behind IPR can transform a region from arid desert to one with forest cover. IPR does not by itself transform a region from desert to forest, instead it initiates and strengthens these natural processes to lead to the desired result. This happens gradually as the environment is modified through steady increases in ET, precipitation and vegetation growth over time. The ultimate goal of the proposed test and research campaign (described here) is to not only demonstrate IPR in action, but to understand how big an impact it can have in solving some of our most pressing problems through favorable environmental transformation.