Reservior & Farm Pond Design

Lesson 3 Identification of Areas Suitable for Water Harvesting

3.2 Parameters for Identifying Suitable Areas

Parameters to be considered for identifying areas suitable for water harvesting include:

• Climatic parameters of the region, especially rainfall

• Hydrology and alternative water resources

• Topography of the region

• Type of vegetation and agricultural production/forestry activities

• Soil types of the region including soil depth and fertility status

• Socio-economic conditions of the community

• National laws and regulations

• Infrastructural facilities available or planned for the area

3.2.1Rainfall Characteristics

The availability of rainfall data collected over years is crucial for the determination of the rainfall-runoff potential of a region. This is particularly true in arid and semi arid regions, where rainfall varies considerably from year to year. However average rainfall can still be used in areas with insufficient rainfall data. Future changes in rainfall pattern expected due to global climate change are also to be taken into consideration.

Rainfall can be measured on site using non recording / recording rainfall gauges to record single rainfall events or the daily total rainfall in the project area. However such data should be used with caution especially when extrapolating the findings to adjacent areas. The elevation of rain gauges from the ground level also affects the amount of rainfall measured. To avoid such discrepancies, the rain gauges should be placed at the same height throughout the project area.

Threshold rain is the depth of rainfall that must fall before runoff starts. It is used in some rainfall-runoff models as an initial value of runoff. Sufficient allowance must be given for the variation of the rainfall in time and space. Apart from the threshold rain that varies with rainfall intensity, the soil type, degree of slope, vegetation cover and antecedent soil moisture condition are other important parameters to be taken into consideration for accurate rainfall measurement.

The intensity of the rainfall is a good indicator of whether a particular rainfall event is likely to produce runoff or not. Of course, determination of the threshold intensity of rainfall that triggers runoff is more difficult to be determined than ascertaining the threshold rainfall depth. Rainfall intensity should also be determined as it is required for rainfall - runoff models. Recording rain gauges can be used for its determination. Rainfall duration can also be determined reliably using a recording rain gauge. This is also an important factor as it is often related to peak discharge in simulation models.

Once these data have been acquired, the most important rainfall parameters to be determined are:

• The relationship between the storm intensity and its duration; and

• The number of storms per year, including their mean standard deviation and probability distribution.

These parameters will then be used to calculate the volume of water available for cropping, possibly by generating synthetic rainfall events for deterministic calculation of runoff quantities.

The temperature regime, air humidity and wind conditions during the cropping period are the other climatic factors which have to be taken into consideration when selecting a certain area for water harvesting.

3.2.2 Hydrology and Water Resources

The hydrological processes relevant to water harvesting practices are those involved in the production, flow, and storage of runoff from rainfall within a particular catchment area. The intricacies of this phenomenon cannot be explained in detail here, but an overview is presented.

Rainfall received in a particular catchment area can be divided into two major components such as the effective rainfall (direct runoff) for water harvesting and the losses. The sources of the losses are:

• Evaporation from the ground

• Water infiltration in the catchment

• Depression storage in the catchment

• Water intercepted by leaves of the plants

In arid and semi-arid areas, extreme fluctuations in both annual rainfall and its distribution during rainy season are considered as the major constraints to agricultural production. In most cases rainfall shows no regular pattern; the wet periods are often followed by marked dry periods. Modeling the rainfall - runoff process in hydrological analysis of an area is very complicated and the model designer must choose the most appropriate model from the existing models or develop one to suit the area under consideration. The lack of meteorological data, suitable topographic maps etc. often creates complications limiting strongly the usefulness of models.

The availability of sufficient runoff from the target area that can be stored to meet the water requirement of the selected crops during the dry periods in between two rainfall events is a good indication of the suitability of the area for water harvesting.

Another factor to be taken into consideration is the availability of other water sources e.g. shallow ground water in wadi beds and renewable ground water from deeper aquifers. These water sources can either substitute runoff water during drought periods or extend the cropping period beyond the rainy season.

3.2.3 Vegetation and Land Use

Vegetation strongly affects the surface runoff. An increase in the vegetative density results in a corresponding increase in losses due to interception, retention, and infiltration, which consequently decreases the volume of runoff. The density of vegetation on a given area can be determined in a variety of ways, but remote sensing is the most advanced and accurate method for large project areas subject to availability of funds. Reflectance of the soil and the vegetation is the indicator of the density of the vegetation in remote sensing.

Land use pattern affects the suitability of land for water harvesting in various ways. Introducing micro-catchment harvesting in areas already under cropping is much easier than transferring farmers into potentially suitable areas. On the other hand, farming activities in catchment areas such as primary and secondary tillage operations reduce the runoff yield significantly due to increase in the infiltration rates. On the contrary, overgrazing removes the vegetation cover which results in higher runoff volumes from the catchment. However, overgrazing entails in most cases a higher soil erosion risk with negative impacts on the water harvesting potential of the region.

3.2.4 Topography, Soil Type and Soil Depth

The suitability of an area for water harvesting depends strongly on its topography and soil characteristics, namely:

• The slope of a terrain which is a decisive factor for any type of water harvesting

• Surface structure which influences the rainfall runoff process

• Infiltration and percolation rates, which determine the movement of water into the soil and within the soil matrix

• Soil depth which together with the soil texture determines the quantity of water that can be stored in the soil.

The topography has a strong impact on infiltration volume and runoff yield. Micro-catchment systems are more appropriate for gentle slopes, whereas macro-catchment techniques can only be established in terrains having significant slope. Further information is given in Chapter 3.

The infiltration rate is the amount of water entering the soil, through its surface, over a given time. Infiltrometers and/or rainfall simulators can be used to determine the infiltration behavior of any soil. The main soil parameters affecting infiltration rate are the texture, structure, and depth of the soil. Vegetation and soil fauna also influence the infiltration rate. Dense vegetation absorbs the kinetic energy of the falling raindrops and thus, reduces the splash erosion and helps increasing the water retention followed by increasing the infiltration rate. A well developed root system after natural decay leaves tubular structures in the soil profile that helps increasing the infiltration rate. On the contrary, a bare soil is dislodged quickly due to its exposure to the direct hit of raindrops and thus, the existing soil pores in the surface are sealed resulting in decrease of infiltration rate.

Initial infiltration rates are higher in dry soils. As the rainfall continues, the infiltration rate declines gradually when the soil pores near the surface are filled up with water resulting in lowering down the hydraulic gradient, the driving force for the infiltration process. In clay-rich soils, the cracks that frequently occur in dry condition get closed as the soil becomes wet.

3.2.5 Socio-economic Condition and Infrastructure

The socioeconomic condition of the stakeholders opting for water harvesting scheme is very much important. Many projects have been abandoned soon after their implementation as a result of the negligence of this very important aspect during planning stage. Key considerations of this aspect include the farming systems of the community under consideration, the financial resources of the average farmer in the area, cultural behaviors and religious beliefs of the people, the attitude of the farmers towards the introduction of new farming methods, the farmers’ knowledge about irrigated agriculture, land property rights, and the role of men and women in the community. The mobility of the populace may also influence the planning decisions.

As in any development project, the existing infrastructures or that will be developed in the future in the same area have to be taken into account when planning a water harvesting scheme.

3.3 Methods of Data Acquisition

3.3.1 Overview

The basic data required for any water harvesting project are presented in Table 3.1. The choice of method used to acquire these data depends not only on technical and financial considerations, but also to some extent constrained by national security and political issues.

3.3.2 Ground Truthing

Field visits to the area where a water harvesting project is to be executed are always necessary. For reliable results, specialists well versed in hydrology, prevailing vegetative condition of the region, and possibly the agricultural practices of the stake holders will be required. Ideally the local experts should be involved with the process. Some parameters may not be directly ascertained from maps, aerial photographs, or even satellite images and so, an inventory of the terrain should be prepared during field visits. Maps and ground truthing are adequate sources of information when the project is to be executed in a small area. However, it is time-consuming and expensive when planned for larger areas or in regional scale.

Table  3.1. Methods for Determining Parameters Relevant To Water Harvesting

Parameter	Used or needed for	Method
Crop water requirements	Maximum dry period, evapotranspiration values of crop	Analysis of meteorological data, plant growth, water stress relations
Water storage capacity of soil	Soil cover, natural vegetation and land use	Assessment of satellite images by computer assisted classification based on ground truth
Accessibility	Distance between water harvesting site and villages	Taken directly from topographic map or by digitizing settlement areas
Type of water harvesting system (micro/macro-catchment)	Terrain slope	Comprehensive distance model
Water availability	Rainfall - runoff relationship	Hydrological analysis/procedures and/or measurements
Sociological, economic, and political considerations	Beneficiaries preferences, resources support, participation, sustainability	Observations, interviews, outcome from nearby water harvesting projects.

 

3.3.3 Aerial Photography

Aerial surveying is a proven technology for extensive data acquisition. Vertical surveys with stereoscopic overlap can be made using large cameras. It depends on the regional or national availability of survey planes, and is cost effective only for large-scale projects. It may be appropriate for water harvesting schemes in regional scale.

3.3.4 Satellite and Remote-Sensing Technology

Satellite and remote-sensing technologies coupled with geographical information systems are the most powerful and reasonably cost-efficient tools used in assessing the potential for water harvesting.

The term remote-sensing is used to describe all the procedures employed in recording information from high altitudes above the Earth’s surface. This can be done from an airplane or satellite. Remote sensing technology can not only be used for gathering preliminary information, but also to monitor and update data continuously at regular intervals.

Various types of information available from a variety of remote sensing platforms are presented in Table 3.2. The principal steps in using remotely sensed data to identify areas suited to water harvesting include:

• Definition of data required e.g. land use, geology, pedology, hydrology, etc.

• Data collection using remote sensing and other techniques

• Data analysis e.g. measurement, classification and estimation analysis

• Verification of the results obtained through analysis

• Presenting the results in a suitable format, such as maps, computer data files, written reports with diagrams, tables, maps etc.

Water, forest, pasture and other features reflect light from the sun differently and yield characteristic patterns in the relation between wavelengths and amount on reflected energy. These patterns can be recognized in the data registered by the satellite. Image classification is based on the assumption that the areas with similar characteristic spectra have similar characteristics on the ground. There are two approaches to classification of the data that are distinguished primarily by their initial assumption. In supervised classification, the ground truth data from direct in-field observations are used to identify the initial parameters used in the classification.

Table  3.2. Information for water harvesting planning from remote sensing systems

Parameter	Satellite type	Type of information	How to obtain
Topography	Aerial photo. LIDAR<IFSAR	Raster data (DEM)	Internet sites, local mapping agencies
Inclination/slope	Aerial photo, LIDAR, IFSAR	Raster data (percent degree)	Derive from DEM
Elevation	Aerial photo, LIDAR, IFSAR	Raster data (meters above mean sea level)	Derive from DEM
Surface roughness	Microwave	Root mean square average	Microwave remote sensing
Soil type	Landsat TM, SPOT, ASTER, others	Polygons of soil mapping units	Interpretation and ground truthing
Soil depth	Ground penetrating radar	Raster (cm)
Soil moisture	Radar remote sensing	Raster (percent)
Land cover/land use	Landsat TM, SPOT, ASTER others	Polygons (classes)	Visual interpretation, image classification and ground truthing
Type of vegetation	SPOT, ASTER	Polygons (type)	Visual interpretation, image classification and ground truthing
Infrastructure	Aerial photo, Landsat-TM, SPOT, ASTER, others	Vector data (points, lines and polygons)	Visual interpretation and ground truthing (local mapping agencies)
Water bodies	Aerial photo, Landsat, TM, SPOT, ASTER, others	Polygons	Visual interpretation and image classification

In remote sensing cartography, the acquired information is first classified in problem oriented categories, and is then mapped in accordance with standard cartographical rules. As compared to approaches using aerial photography and ground truth, less effort is required to process the remotely sensed data because certain stages of the analysis can be assessed on the monitor to elaborate the statistical evaluations. Since the data gained through this system is in digital form, it can be translated to adjacent scenes with the consideration of the existing illumination differences without the need to carry out field investigations. Since the remotely sensed data are in digital form, it can be further processed and even linked to other compatible data sets. 

3.4 Tools

3.4.1 Maps

Maps may be the only means of acquiring data in some countries from Google earth images. Two types of maps such as topographic and thematic have been used commonly in gathering land related information.

Topographic Maps

A topographic map represents the features of an area in an analogue form. This type of map can be found in many regions of the world. They can be digitized and incorporated into a GIS database.

Thematic Maps

Thematic maps represent specific types of information e.g. soils, rainfall or temperature isohyets, vegetation types, etc. These maps present source information in classes. It should be noted that a degree of inaccuracy exists in the way the classes are defined. For instance, a continuous phenomenon such as soil or vegetation type is mapped as homogenous map units with sharp boundaries, whereas the actual circumstances on the ground vary within each map unit; this may affect the project results significantly.

3.4.2 Aerial Photographs

There are archives of black and white aerial photographs in many parts of the world, but their usefulness depends on the age and scale of the images and the specific purpose for which they were taken. Color infrared photographs can be used to differentiate vegetation types.

3.4.3 Geographic Information System

A GIS is a computer based system used to capture, store, edit, manage, and display geographically referenced information, including spatial and descriptive data. Spatial data deal with the location and shape of various features and the relationship among them. Such features as topography, water resources, soil types, land use, infrastructure and administrative boundaries can also be combined in a GIS.

Descriptive data describe the characteristics of these features. Thus a GIS serves as a tool for representing the real world. GIS can be used to help policy makers in identifying and prioritizing appropriate rainwater harvesting interventions.

Keywords: Rainwater harvesting, Hydrology, Remote sensing, GIS

Suggested Reading

• Owesis, T. Y., Prinz, D. and Hachum, A. Y. (2012). Rainwater harvesting for agriculture in dry areas. CRC Press publication.