Assessment of environmental quality using Remote Sensing (RS) and Geographic Information System (GIS) techniques
Environmental quality is the group of properties and/ or characteristics of environment either generalized or local which has a direct impact on all living organisms. All three components of environment; soil, water and air are influenced by environmental quality which is helpful in risk assessment of these components. Environmental risk assessment provides guidelines to identify the causes of environmental pollution and their remediation approaches. Geographic Information System (GIS) and Remote Sensing (RS) are effective tools of environmental risk assessment for the determination of soil, water and air pollution. These techniques provide better interpretation of collected data in the form of images or maps to grab the attention of common people for the remediation of polluted area. Heavy loads of raw city effluents are continuously polluting the soils with heavy metals and soluble salts. Farmers are utilizing these effluents to grow food stuff like hot cakes ignoring its long term deteriorative effects on soils and ecosystem. Due to these malpractices, there is a steep buildup of toxins in the soils which are serious threats to food security and sustainability. Similarly, soil salinity and water logging are also spreading on a mass scale and shrinking the cultivable area and causing the motivation for urbanization. Majority of our farmers are resource poor and have very small land holdings. Therefore, they opt the easy option of sale out these anthropogenically degraded soils to industrialists. There is an urgent need to motivate and make the farmers aware about the seriousness of the problem not only for the existing but also for their future generations. The 1st constructive step in this regard is to identify the pollution hit areas starting in the suburbs of the big cities. Numbers of international projects have generated the scattered data on heavy metal, salinity and water-logging. The need of the hour is to make an effective utilization of this data by interpreting it in a proper way using some latest modern techniques. So that we can identify and prioritize the rectification/ remediation of the pollution hit areas. GIS and RS can direct us towards solution of the problems by identifying and/or quantifying extent of problem.
The GIS was developed in 1960 in Canada to collect analyses and store data but with passage of time it was modified and flourished into a very useful tool to monitor earth, its environment and mapping/modeling its physical features. It deals with position of any object on the earth and uses computation and analysis of data acquired from ground and its relationship with the position of object obtained from Global Positioning System (GPS). To find actual extent of problem we need to collect samples from study area along with latitude and longitude value with handheld GPS. These points then are displayed in any GIS software and different interpolation (Estimating value of an un known site based upon know site data) operations are performed like Kriging, IDW, Natural Neighbor, Spline, and Trend etc. It is an efficient way of monitoring soil, water and air pollution. Soil degradation over time, soil salinity parameters (electrical conductivity (EC), sodium adsorption ratio (SAR), and pH), soil nutrient status (macro and micro nutrients), soil physical properties (soil texture, soil strength, and soil permeability etc), soil pollution (heavy metal pollution, organic pollutants), water and atmospheric pollution (green-house gases, dust, smog, smoke and particulate matter) can be monitored and mapped using these models and techniques. Most frequently used GIS software’s are ArcGIS developed by ESRI, GeoMedia by Intergraph/Hexagon geospatial, QGIS, GrassGIS, SagaGIS, Map Info and Global Mapper. GIS is incomplete without ground truth data (sampling sites data) it can only interpret, and model based upon ground data so intensive sampling and field survey is necessary.
Unlike GIS, RS allows to extract information about any physical object without being in direct contact with it. It involves satellite remote sensing (SRS), unmanned aerial vehicle (UAV) remote sensing, Kite remote sensing and handheld instrument for remote sensing. Basic principle of remote sensing is, light radiation in different wavelengths being reflected from ground/air/water/object surface collected by a RS device either a camera and/or sensor. Unique features of an object have their unique behavior of light reflection called spectral signature. For example, water reflects blue or no light so it shows blue or black in satellite imagery. Muddy water will appear brown, bare soil will be greyish, saline or calcareous soil will appear white or grey, vegetative area will be red or green (based upon reflection), cities appear grey, atmospheric features also show their respective reflectance signature. Along with visible spectra RS sensors can detect very long or short wavelengths (Near infrared NIR, Shortwave Infrared SWIR) which are invisible to naked eye. Soil physical and chemical properties alter these light signatures and we can couple change in these reflection with changing soil/ atmospheric parameters and classify remaining area accordingly. For satellite remote sensing of soil heavy metal, organic pollutant or any other pollution active or passive satellite remote sensing data is used. For soil and water pollution monitoring data from following satellite can be used; freely accessible satellites are Landsat 4-5,7-8, sentinel and Moderate Resolution Imaging Spectroradiometer (MODIS) and high resolution commercial satellites are, SPOT, IKONOS-2, OrbView3. For atmospheric composition and pollution monitoring, data from AQUA, AURA, CALIPSO, ACE, and CartoSat can be used. For green house gas emission from earth and through human activity GOSAT and OCO2 satellite data can be used. Satellite data for these satellites can be accessed using their direct weblink or using websites like https://earthobservatory.nasa.gov/, https://earthexplorer.usgs.gov/ and https://earthdata.nasa.gov/.
Researchers working at Institute of Soil and Environmental Sciences (ISES), University of Agriculture Faisalabad (UAF) are using GIS techniques to monitor soil salinity, heavy metal distribution in agriculture soils and road side dust, suspended particles in air and water quality since long. With recent advancement and free access to international satellite community data anyone can keep an eye on his surroundings and can play a role to save our environment.
This article is collectively authored by Muhammad Ashar Ayub, Hammad Raza Ahmad, Hinnan Khalid and Muhammad Zia-ur-Rehman.