The vital role of earth observation satellites for monitoring water resources and agriculture in the MENA region
The problem of plants cultivated in soils with high salinity is the concentration of toxic ions in the root zone, affecting water uptake and transpiration. An experiment at ICBA suggests a genetic component in reaction to salinity.
Author: Adla Khalaf. Source: Biosalinity News
Determining changes in water resources, especially those beneath the ground, is difficult and results in decision-making based on limited evidence. Earth observation satellites can help with in some areas of natural resource management as they are designed to observe different parts of the land surface, atmosphere, biosphere, and oceans of the globe.
One of satellite observation mission that could prove really helpful in managing water resources is the Gravity Recovery and Climate Experiment (GRACE) system. This satellite based sensing system measures orbital variations caused by changes in Earth’s gravity field, which are directly related to changes in terrestrial water storage (TWS) in vertical columns, integrated from the earth’s surface down through the base of the water table.
GRACE provides data on TWS anomalies that are not available to hydrologists by any other practical mean. It gives hydrologists the ability to close the terrestrial water storage budget by providing a quantitative estimate of the total integrated water mass variations over time and over large-scale hydrological and hydro-geological applications at spatial resolutions of at least 150,000 km2 with a precision approaching 1.5 cm of water equivalent thickness.
While most satellite remote sensing missions use radars or radiometers to measure various wavelengths of light which are reflected or emitted from Earth, GRACE does not look down. Instead, water storage changes are achieved by using a microwave ranging system that continuously measures changes in distance between the twin satellites in the polar orbit at 500 km altitude spaced at approximately 220 km.
The satellite orientation is measured using twin star cameras and position reading derived from a GPS receiver (see Figure 1). The spatial and temporal variations in the earth’s gravity field affect the orbits of the twin satellites. These differences are observed as changes in the distance between the two spacecrafts reflecting in the time-of-flight of the microwave signals transmitted and received between the two spacecrafts. The changes in time of flights are continuously measured by tracking the phase of the microwave signals that are ingested into a massive regression equation to churn out monthly level-2 gravity field solutions. The effects of atmospheric and oceanic circulations are removed using numerical model analyses.
The level-2 products can be converted to water mass anomalies (deviations from the series mean) using averaging kernels which have been defined for the regions of interest.
There were some positive trends as the map shows, in Mauritania, Morocco and Sudan. The highest declines derived from the GRACE imagery are in the Levant, Egypt, Saudi Arabia, and across the confined aquifers in North Africa.
The average total water storage changes per year for the study period are -0.59 cm, -1.94 cm, and -0.04 cm in the Arab Peninsula, Levant, and North Africa, respectively (Figure 4) that are equivalent to 207.8 km3, 160.1 km3, and 39.3 km3 during the study period. These volumetric values are huge and represent in many cases major depletion of non-renewable resources to predict water scarcity problems in the region.
Results for the MENA region give important insight. GRACE terrestrial water storage anomalies were calculated and applied over the MENA region from January 2003 to January 2014. Variations relative to average values were then plotted and mapped (See Figures 2 and 3). Linear trend shows a decline in water storage at an average rate of -2.57 cm year-1. Whilst this does not sound much especially when compared to individual well measurements, this value refers to the average over a very large area which highlights of how much water has been lost.
Another area of related remote sensing activity at ICBA has focused on deriving up-to-date values for irrigation in the MENA region, as this is an important factor affecting the water balance in the region.
Few countries in the region have up-to-date assessments of their areas under irrigation and so water use. Accurate geospatial information on the extent of irrigated land is required to improve our understanding of agricultural water use, local land surface processes, conservation or depletion of water resources, and components of the hydrologic budget.
A new MENA irrigation map has been developed to identify irrigated agriculture at 250 m using MODIS (Moderate Resolution Imaging Spectroradiometer) datasets. MODIS is considered a key instrument aboard the Terra and Aqua satellites viewing the entire Earth surface every 1 to 2 days, acquiring data in 36 spectral bands. With these, two bands are imaged at a nominal resolution of 250 m at nadir, with five bands at 500 m, and the remaining 29 bands at 1 km.
The global MODIS Normalized Difference Vegetation Index (MOD13Q1) is designed to provide consistent temporal and spatial vegetation conditions. It is provided every 16 days as a gridded level-3 product in Sinusoidal projection.
Data acquired in 2012 has been obtained from the Land Processes Distributed Active Archive Center (LP DAAC) covering the 32 tiles extending over the whole MENA region. Seven hundred and thirty six 16-day composite period images are downloaded and stacked using a specialist form of computer library commands to create a 23-band multi-temporal image. Training sites are delineated using Google Earth and medium resolution Landsat images, and categorized into irrigated and non-irrigated sites. Image classification is performed using the Support Vector Machine where classified tiles are then arranged in mosaic form as shown in Figure 3.
The new irrigation map has many uses including updating policy makers on irrigation extents in the region and in their countries. It is also a vital input to any subsequent water and crop modelling.
This helps modellers derive more accurate representations of water use and so the resource balances in any area. It is also important input to climate adaptation planning as irrigation, crop possibilities and future water availability can be modelled with this information.
The ultimate aim is that ICBA becomes a knowledge hub to provide data and analysis of climate change, availability of water, and agriculture, using the tools developed through the Modelling and Monitoring Agriculture and Water Resources Development Program in ICBA.