Water harvesting can be defined as the collection of runoff generated by precipitation to provide water for human, animal, or crop use. This method is a small-scale water resources technique that collects and stores rainwater through structural measures, and regulates and uses it for domestic and/or agricultural production purposes. The main purpose of water harvesting is to provide a reliable water supply in areas where groundwater and surface water resources are absent or where their development is not economical.

History

Water harvesting is a common application technique used by different civilizations for approximately four thousand years. In the past, it was mostly used for drinking purposes or agricultural applications. For example, Egyptians produced storage tanks with capacities ranging from 200 m³ to 2000 m³, some of which are still in use today. Rainwater harvesting practices in Thailand date back approximately 2000 years. It is known that the roof catchment system was used in Roman times, and Roman houses and cities were designed to utilize rainwater as the primary source for drinking and domestic use since 2000 B.C.. In Israel's Negev Desert, in places receiving 100 mm of annual rainfall, tanks were used to store runoff from slopes in agricultural and residential areas. The basic rainwater collection and storage methods used today are not very different from those used in the past; however, the technological and structural material aspects of the systems and the ways stored water is used have changed.

Importance

The effective use of water, one of the most critical life elements in the world, is gaining more importance day by day. With the continuous increase in population and climate change, the consumption rate and usage patterns of water resources have become a significant issue. The fact that no artificial substance will be found to substitute for water in the future further increases the strategic importance of water. In this context, water harvesting stands out as an important solution method by offering benefits such as reducing the pressure on water resources, increasing the total amount of irrigation water, and preventing soil erosion. In arid and semi-arid regions where rainfall is insufficient and irregularly distributed, leading to widespread water scarcity, water harvesting structures have the potential to increase the productivity of cultivable areas by reducing the risk of crop failure and ensuring an increase in yield.

Basic Components

Water harvesting systems basically consist of three main components:

Catchment Area (Collection Area): This is the part where precipitation is collected and is also referred to as the runoff area. This area can be as small as a few square meters on roofs or as large as several square kilometers.
Storage Area: This is where the harvested water is kept until it is used by humans, animals, or plants. The storage medium can be underground (soil, sediment, cistern) or on the soil surface (tank, reservoir, pond).
Target Zone: This is the user of the harvested water. In domestic use, the target is people or their needs, while in agricultural production, the target is plants or animals.

Classification and Techniques

Water harvesting methods are generally classified based on the size of the catchment area into micro-catchment and macro-catchment (and floodwater) systems.

Micro-Catchment Systems

These systems involve the smallest units in the classification, both in terms of operation and size. Runoff is usually provided through artificial slopes or channels. They are applied for trees where annual rainfall is 200 mm, and for annual plants where it is 300 mm.

Farm Systems

Inter-row Water Harvesting: Used on flat land and gentle slopes (0%-4%) with soil depth of at least 1 m. It is based on creating ridges in the inter-row spaces where plants are not cultivated. It is the only water harvesting technique that can be used on completely flat lands, and its construction can be fully mechanized.
Negarim: These are small diamond or rectangular-shaped grid earth bunds surrounded by constructed artificial embankments called bunds, applied in small runoff catchments. They can be built on almost any slope, but there is a risk of soil erosion on slopes over 10%. Recommended for regions with 150-500 mm annual rainfall. It also prevents soil erosion.
Meskat: A term commonly used in Tunisia. Suitable for lands with 200-400 mm annual rainfall and a slope of 2% to 15%. It has one catchment area but can feed multiple cultivated areas (target zones). Used only for growing trees.
Contour Terraces (Bench Terraces): Constructed on steep lands with a slope between 20% and 50% (some sources say up to 60%). It also aims to protect the soil against erosion. Cultivated terraces are generally built flat and supported by stone walls. It can be used in areas with annual rainfall between 200 and 600 mm.
Small Pits: Consist of pits dug to a depth of 5 to 15 cm and with diameters between 0.3 and 2.0 m. Also known as the "Zay system". Used for growing annual crops and seen in regions with 350-600 mm annual rainfall, on flat land or slopes approaching 5%.
Contour Ridges (Bunds): Created along contour lines varying between 5 and 20 m. The area between the ridges is cultivated, while the rest is the catchment area for water collection. It can be built on lands with slopes varying from 1% to 50%. Used in areas with 300-600 mm annual rainfall.
Eyebrow Terraces (Planting Pits/Microbasins): A semi-circular bund form using stones to support the downhill side. Usually found on slopes and can be used on slopes up to 50% (some sources say 1-50%). Productive in regions with annual rainfall values of 200-600 mm.
Semi-circular Bunds: Seen in semi-circular or trapezoidal shapes, with the ends of the ridges along contour lines adjusted to face the slope. Used for rangeland improvement or fodder production in regions where annual rainfall exceeds 300 mm. Applicable up to a maximum slope of 15%.
Vallerani Micro-catchment System: A fully mechanized system for preparing small micro-catchments for afforestation. Applicable in areas with 200-600 mm annual rainfall and slopes between 2% and 10%.
Hillside Micro-catchments: Rectangular micro-catchments that provide sufficient water for a single tree or shrub. Applied on 1-50% slopes and viable in areas with 200-600 mm annual rainfall.

Rooftop Systems

Rain harvested from roofs is used as drinking water, especially in rural areas. 80-85% of the falling rain can be harvested and stored. The system is based on the principle of collecting precipitation falling on the roof surface, conveying it via gutters to a tank on the ground surface or an underground storage, and storing it there. It is also used to recharge groundwater. Harvested water can also be used for needs such as garden irrigation, car washing, toilet flushing, and cleaning.

Macro-Catchment and Floodwater Systems

These systems have larger catchment areas than micro-catchment systems.

Valley-Bed Systems

Small Farm Reservoirs: Small ponds built by farmers whose lands are in valleys to store all or part of the surface runoff flowing along the valley. Their capacities can range from 1000 m³to 500,000 m³
Wadi-Bed Cultivation and Jessour: Systems based on the principle of blocking or slowing down the flow of water in wadi beds, allowing it to infiltrate and be stored in the soil profile. Common in gently sloping wadi beds. The Jessour system consists of a barrier, a terrace, and a collection area.

Off-Valley Systems (Floodwater Harvesting):

These systems are based on the collection of irregular seasonal streamflow.

Water Distribution Systems: Diverting water flowing naturally from the valley to storage areas (usually the plant root zone) for accumulation.
Large Bunds (Rabla): Used to harvest water coming from large mountains and wide surfaces during the rainy season. The distance between embankments is about 10-100 m, and the bund height is expected to be at least 1 m.
Water Tanks: Water collection areas created to meet the water needs of humans and animals, with capacities typically starting from 1000 m³
Cisterns: Systems used as a solution to water shortages in residential areas, usually built underground and waterproof. Water flowing from the roofs, courtyards, or terraces of buildings in the city is channeled into cisterns. The Yerebatan Cistern in Istanbul is an example.
Hillside Runoff Systems (Floodwater Diversion/Spate Irrigation): Structures that cause water flowing from the valley to leave its natural course and direct it to agricultural lands. In this system, water is usually stored in the crop root zone.
Stone Bunds, Large Semi-circular Bunds, Trapezoidal Bunds: Also considered macro-catchment techniques.

Advantages

Water harvesting has many direct and indirect benefits:

It provides a reliable water source, especially important in regions with limited water resources.
It increases agricultural production and reduces the risk of drought.
It reduces soil erosion and sedimentation.
It increases water storage in the soil and soil fertility.
It helps recharge groundwater aquifers.
It can reduce flood risk.
It is often low-input and not difficult to apply.
It generally does not require pumps or energy input.
Environmental benefits include combating desertification and supporting ecosystems.
It can contribute to rural development, reduce migration, and create new job opportunities.

Limitations and Challenges

Water harvesting systems also have some limitations and challenges:

It is dependent on rainfall, and the irregularity of rainfall can affect the system's effectiveness.
The system can be damaged during heavy rainfall.
The water collection area cannot be used for agricultural applications.
It may be programmed to produce only at a low level compared to other irrigation systems.
The lack of a well-established scientific method can be a challenge.
Potential conflicts may arise between people located downstream and upstream.
There may be a risk of harming local fauna and flora.
Difficulties may be experienced in the implementation of large-scale systems and structures.
Installation and maintenance costs can be high for some techniques (e.g., small farm reservoirs, contour terraces).

Planning and Implementation

Careful planning and appropriate technique selection are important for a successful water harvesting project:

Rainfall Characteristics: Data such as the amount, duration, frequency, intensity, and distribution of rainfall in the region play a critical role in system design. Reliable and long-term rainfall data are needed.
Catchment Characteristics: The catchment area, slope, soil type, vegetation cover, and surface characteristics (e.g., roof material) affect the quantity and quality of water collected. The runoff coefficient indicates how much of the total precipitation falling on the collection area can be collected.
Water Demand: The water demand of the place where the collected water will be used should be determined and compared with the harvest potential.
System Component Design: The collection surface, conveyance system (gutters, pipes), first-flush diverter, storage tank, and, if necessary, water treatment system must be correctly designed and sized. The size of the storage tank is one of the most important factors affecting cost.
Water Quality: The quality of the collected water is important depending on its intended use. Especially if it is to be used as drinking water, roof material, first-flush diversion, and filtration/disinfection processes are of great importance.

Water Harvesting in the Context of the Global Water Crisis

Globally, water resources are progressively diminishing and becoming polluted due to increasing population, industrialization, agricultural irrigation needs, and the adverse effects of global climate change. Many countries are facing water stress or scarcity. Turkey, too, is in a position of being a country prone to water scarcity in terms of per capita available water. Under these conditions, alternative and sustainable water supply methods like water harvesting hold an important place in strategies for the wise use and conservation of water resources. Water harvesting has the potential to offer local solutions to this global problem by providing support to existing water resources.

Water Harvesting