C. J. Jagadeesha
Regional Remote Sensing Service Centre
Indian Space Research Organisation
Bangalore Tel: 6661003
Advantage of using remote sensing data for hydrological modelling and monitoring is its ability to generate information in spatial and temporal domain.
Remote Sensing (RS) data and Geographical Information System (GIS) play a rapidly increasing role in the field of hydrology and water resources development. Although very few remotely sensed data can be applied in hydrology, such information is of great value since many hydrologically relevent data can be derived from remote sensing information. One of the greatest advantages of using RS data for hydrological modelling and monitoring is its ability to generate information in spatial and temporal domain, which is very crucial for successful model analysis, prediction and validation. However, the use of RS technology involves large amount of spatial data management and requires an efficient system to handle such data. The GIS technology provides suitable alternatives for efficient management of large and complex databases. Information from satellites is becoming more and more important for environmental research; an important part of this information concerns water – an element most essential for man, its phases and peculiarities. If the Earth surface image are available, the information obtained may be considered as a spatial model of a natural event. The problem is to interpret the model, to measure its basic parameters, to discover interrelations between the events and to determine changes introduced to the environment. Particular problems correspond to the different spatial scales. As for surface water problems, high resolution imagery from IRS series, Landsat series and SPOT satellites provide basic information for the parameterisation of conditions for different hydrological regimes and for water resources evaluation.
Issues in water management
The various identified issues in water resources development and management situation in the country can be broadly classified as
- Surface water management issues.
- Ground water management issues.
- Integrated river basin management issues, encompassing environmental issues and institutional issues.
Issues connected with surface water management are :
Theoretically, we have enough renewable water in our annual rainfall. However. considerable resourcefulness, innovative skill, lack of community participation and local involvement in design and management are required to use that water to meet our requirements.
- Attitude that water is the ‘basis of life’ and is ‘beyond price’ – The lack of economic appraisal of water projects ; downstream cost of upstream benefits, internalisation, etc.
- Front end problem whereby the incentives appear to be to build a “monument” and move on to another project
- Most planning is done incrementally and is driven by the need to find solutions to relatively immediate and specific problems, often requiring an unique institutional framework.
- The issue of assurance is addressed in the context of what are the institutional characteristics of the successful projects else where.
- Water conservation by controlling water loss, recycling & reuse, improving agriculture water use by water control institutions.
Issues connected with ground water management are :
- Aquifer and draw down issues – like depressed water table – interaction between shallow and deep aquifers and imbalances between fresh water and salt water interfaces.
- Ground water support service issues – technical performance of ground water utilisation in improving command area, recovery of water charges; ownership issue of individual versus group management and public versus private.
- Sustainability issues in water surplus areas – like balancing surface water and ground water development for optimal utilisation etc.,
- Sustainability issues in water deficit areas – like planning for water conservation, water quality management improved recharge, conjunctive use ; the progressive deepening of wells (over – exploitation) over put small farmers at a disadvantage.
Issues in integrated river basin management
- Proper identification of development objectives and their prioritisation with the technological choice socio-economic milieu -regional level optimisation models using multi objective approach providing scenarios for short listing and final selection.
- The concept of information system (or infotech highway) which will have relevant hydrologic and non-hydrologic data at appropriate resolution from a judicious mix of ground and remote observations – i.e the concept of Natural Resources Accounting for water.
- The sustainable water development faces several obstacles like a) fragmented sector policies b) weak or non-existent institutions and inadequate co-ordination among sector agencies c) lack of adequately trained and motivated manpower d) use of technologies inappropriate for developing country conditions e) lack of community involvement f) problems of resource mobilisation and utilisation and cost recovery etc.
- Representative issues in water quality management including identifying levels of management viz passive, active, proactive etc; availability of dilution water, costs of waste treatment vs disposal, allocation of environmental costs and benefits for biodiversity disturbance, submergence of forest, eutrophication etc.
In river basin studies, the applications of averages for hydrological parameters are meaningless since supply and demand are not evenly distributed over time and space in river basin. Following are to be done for a rigorous assessment. An intervention model for average ten day stream flow forecast and synthesis. The digital simulation of a drainage basin to convert rainfall into abstractions and surface runoff. The exact and approximate evaluations of the storage distribution in a water reservoir are not rigorous. The likely performance of water resources systems is often described by the mean of various benefits, pollutant concentrations or some operating variable. Take for instance the case for a groundwater system. Quantum of water which can be extracted economically from under ground every year is generally reckoned in engineering circles as groundwater potential. This implies that water extracted in any year should be returned to the aquifers in the form of recharge from the succeeding precipitation or recharge, so that over a cycle of two, three or five years the groundwater table does not go down progressively. In these cases also simple averaging do not help in resources assessment.
With RS technology being the major source of spatial data on natural resources and when combined with other data sets – conventional maps, GPS data, census data and so on, will offer an information packet, hitherto unavailable to the decision maker. With the availability of information packet, GIS technology and applications will impact most of our resource management and decision making in a major way.
Remote sensing based water quality assesment
Remote Sensing with its unique capabilities of synoptic coverage, repetitively and capturing features of inaccessible areas like reservoir water contours and crop water use zones (CWZ) has been effectively used in monitoring reservoir water use (water budgeting) and irrigated crop water use to assess the performance of main system management aspects of large scale irrigation systems.
Reservoir water use budgeting in a typical multipurpose reservoir
The following procedure was used to arrive at actual/realisable amount of water that is available for use in the reservoir-command during the time interval of satellite overpasses. Reservoir water spread area (water contours) obtained from satellite data every fortnight during rabi/summer season are used in assessing water quantity available between satellite overpasses. Water budgeting is carried out.
Reservoir working tables can thus be updated once in 22 days/15 days – till such time as all the components of water balance into a “stochastic reservoir” are arrived at using time series/intervention models and “phase” of lake budget is calculated. The above procedure results in actual amount of water that is available for use in the command during the time interval of satellite overpasses.
The results of the above procedure helps in assessing the following in a river basin/part of a river basin. The possible effect of negative inflows in a river as arrived at in daily water budgeting tables used in operation of a reservoir. The above has a bearing on upstream water utilisation (being less than actually recorded utilisation).
The working tables used for water balancing at sub-basin level gets a check on their parameter values if satellite data is used at appropriate times. The check on carry-over storage at basin level will become precious for correct valuation of water utilisation at both upstream and down stream users.
Remote sensing derived crop water use zones
The characteristic spectral signature of vegetation in the visible and near infrared bands was recognised early in the history of remote sensing. It is perhaps surprising that satellite remote sensing techniques have found few “real world” application in agriculture (irrigated or dryland both). Satellite systems that have the resolution to recognise individual fields are constrained to repeat cycles that are too long to characterise the growing season of a typical annual/seasonal crop. The problem becomes much more complicated in small land holdings, having mixed cropping and multiple cropping systems with varied agricultural management practices.
A water quantity variable namely crop water use zone (CWZ) has been delineated using remotely sensed IRS images. It includes consumptive use by crops and land. Three CWZ describing heavy, medium and light water use zones in a CCA can be mapped with it. These CWZ have been used to define four irrigation performance indicators which represent the uniformity, adequacy, equity and dependability of irrigation water distribution at main system level in a large scale irrigation system.
Using the above, the following can be assessed which helps in river basin/sub-basin water use assessment in a rational manner. The water utilisation (not water requirements of crops) in each command (a part of sub-basin) can be had from satellite imageries when one establish relations.
The satellite derived water utilisation helps in assessing irrigation performance of each command area projects with water as the main input. These maps of water utilisation helps in planning for better conjunctive use. Farmers participation in irrigation management increases because the current method(s) of measuring discharges only at distribution head and gauging water height at sub-distribution or pipe-outlet level – and a whole of lot of statistics for the season wise distribution of water are not comprehensible by farmers. A map depicting water use is going toincrease their awareness and hence better participation. It helps in comparing financial and irrigation performance of divisional/sub-divisional jurisdictions.