Permaculture Earthworks

Excerpt from The Permaculture Earthworks Handbook

Chapter 4

The potential for overharvesting

Water is an issue that starts conflict, and greater warfare over water is a major concern in a world where the demand for water is increasing at twice the rate of population increase. The issue becomes even more sensitive in semi-arid and arid regions.

Permaculture was born in Australia, and many of the water-harvesting earthworks employed by permaculture were and are widely used there. Since the 1970s, there has been an increase in the deployment of farm dams. Since that time, there has also been a recorded decrease in rainfall of 15 percent. These two factors have led to a decrease in streamflows throughout the nation. To make matters worse, the impact of dams on streamflow is greater during dry years than during wet ones. This has, understandably, led to conflict. As a result, the establishment of new dams is coming under stricter regulation.

In general, regulations around water-harvesting earthworks exist for two reasons. Firstly, water is a public good. You have a right to water on your site, but so do people downstream. Water is a public good, and if you claim it as your private property, you are running afoul of both the law and universal human rights. As seen in Chapter 1, claims to water upstream have destroyed entire ecosystems. Water is yours to make use of, then pass on for others.

A study of the Yass River in the Murray-Darling Basin (Neal et al., 2002) found that for every 1 megaliter (264,000 US gallons) of farm dam storage there was a 1.3 megaliter (343,000 US gallon) reduction in streamflow. Similar reductions have been found in other studies, as well. This reduction in streamflow increases the duration and occurrence of periods of low streamflow and zero streamflow. That the rate of reduction of flow is greater than the volume of the dam is not surprising. Not only is the captured water used for irrigation of crops or livestock, there are also significant losses in dam volume due to evaporation. In humid areas of Victoria, 10 to 20 percent of a dam’s volume is lost to evaporation, and that figure jumps as high as 70 percent in the drier areas of the state.

Dams are associated with a lowering of streamflows, though the problem is more complex than simply the introduction of dams. For the sake of agriculture, much of the native eucalyptus forests have been cleared. In one study looking at catchments in the Kent River in Western Australia, just 36 percent of the native vegetation remained in the study area. The rest had been cleared, largely since the 1940s, with the land now used for the production of annual cropping. This is significant in several ways. The clearing of forests reduces rainfall and occult precipitation. (See Chapter 7.) It sees a reduction in hydraulic conductivity, which means greater runoff of the rain that does fall. Farm dams established for crop irrigation are drained down in the service of crop irrigation. This means that dams are often emptied, so more of the total annual precipitation is taken out of the local hydrological cycle. By contrast, dams for livestock irrigation are not drawn on as much and have a tendency to remain fuller than dams for irrigating crops. This means that less water is used and more water overflows in the spillway to return to the watershed. The impact on streamflows for livestock dams is, in general, less than that for crop-irrigation dams.

The situation around dams is not cut and dried, even for more sensitive drylands. As you will see in Chapter 6, check dams and even small-scale concrete dams in semi-arid regions can help to re-green areas. Streamflow volumes are one measure of what is happening in the hydrology of a landscape, but it is not the total picture. Semi-arid Andra Pradesh, India, was traditionally a tropical monsoon region, but biotic pressures have reduced the land to a semi-arid state. The major rain events come during the monsoon season. With the land as barren as it is now, there is tremendous runoff that very rapidly rushes off the land, through river ways, and out to sea. The monsoon rains make up the bulk of the water available for both people and wildlife to use through the course of the year.

Adjacent to a site highlighted in the Indian case study in Chapter 7 was a small concrete barrier dam holding around 35 million liters (9,246,000 US gallons), built by the Rural Development Trust, a local NGO. This dam unquestionably reduces total streamflow volumes. It also increases residency times for water in the landscape. The base of the dam has a steady throughflow, which has made possible the establishment of a mango orchard and rice paddies that employ dozens of local townspeople. While it is true that barrier dams have a greater impact on river systems, the scale of this dam is quite small, and it is high in the watershed. Additionally, its long and narrow shape is sheltered between two hillsides , reducing wind exposure, which in turn reduces evaporation. Yes, it is reducing total streamflows, but it is providing a livelihood for the neighboring hamlet, and it is an important refuge for local wildlife.

In both Andra Pradesh and Australia, the greatest impact on the hydrological cycle has been the removal of forests. The source of the water problem is deforestation. Blaming the increasing aridity solely, or even mainly, on water-harvesting systems is ignoring the major culprit. Indeed, earthworks can be a beneficial, or even vital, ingredient in reforestation efforts. Inappropriate small-scale earthworks can compound water problems but are not the sources of them. Increased development of silvopastures (systems combining trees and pastures) for livestock operations and alley cropping for annual crop production need to be increasingly employed in drylands in order to combat aridity.

Dams can decouple catchments from the watershed, which contributes to reduction in streamflows. Dams make a contribution to groundwater, though this is not well studied, and it is reasonable to expect that the contribution is not very significant. For a storage dam, effort is put into minimizing leakage, meaning that infiltration of water into the soil is discouraged. In the case of traditional India johad, however, infiltration is the goal. In a johad, runoff is intercepted and held in place specifically so that it has time to infiltrate the soil and recharge groundwater. There is a similar outcome in the natural world, in which beaver dams contribute to groundwater recharge. Though the beavers’ motivation is not to recharge groundwater, recharging is nevertheless a byproduct of their damming of small streams. Higher groundwater levels are recorded as a result of beaver dams, which contributes to the consistency of streamflow downstream. Depending on the design of a dam, there is potential to make significant contributions to groundwater, which can positively affect streamflow.

It is worth mentioning, however, that although the usefulness of dams is greatest in times of drought, these are also the times in which they have the greatest impact on the hydrological cycle. If, for example, your dam is three quarters full, and your dam’s catchment collects half the dam’s volume in rainfall, then the excess one quarter flows downstream to others over the spillway. If, however, the dam is only one quarter full, and the same volume falls, all the water will be captured. The impact of dams is greatest in drier times.

An effective water-harvesting system that maximizes water availability, both on site and downstream, will seek to slow, not stop, the flow of water. Infiltration is a prime strategy in this approach. This makes water biologically available for soil life and for plants as well as recharging groundwater, making more water available in wells and providing better flows in springs. Streamflow is important, but it is a deceptive measure of riparian health. Maximized streamflows are not, and should not be, the goal. If you completely denude a landscape of all vegetation, runoff will maximize. As a result, streamflows will increase as well. In this scenario, streamflow volumes will fluctuate wildly with precipitation. Denuding enough of the landscape will lead to decreases in rainfall and wilder streamflow fluctuations. Runoff water will also be warmer, reducing its dissolved oxygen content, and will carry more sediment. This approach has been inadvertently used time and time again in human history. The floods of ancient Sumer and the gradual desertification of Mesopotamia are our first recorded instance of this, though the pattern has been repeated throughout the world. Clearly, maximizing streamflow is not the goal. Rather, we should seek for consistency of streamflow.

Water-harvesting earthworks are neither a panacea nor a scourge. The effects of hydrology on ecology are nonlinear and are subject to complexity. This means that we cannot fully predict what the outcome will be based on the initial inputs of the system. There is no simple, linear equation that says dryland areas plus dams equals downstream drought, or success for that matter. The human mind has a tendency to gravitate toward dichotomy. We notice a success with earthworks, and they become good things in our mind. Or we see a negative outcome, and earthworks become harmful human intrusions on the land. Landscape hydrology and ecology are too complex to make this kind of reduction. This is why the design process is so important. We need to develop responsible goals for our projects, then adjust to the feedback available. The first guideline for permaculture is to care for the Earth. Our goals need to be aligned with this guideline, and we need to check feedback for the impact of our projects to ensure that they are living up to that guideline.