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Campaign (EISAC’89) which took place at a site in the Somerset Levels during May 1989. The specific focus 
of interest was an experimental grassland site on Tadham Moor managed by the Institute of Grassland and 
Animal Production (IGAP) and the Institute of Terrestrial Ecology (ITE). The site comprised 20 ha of 
unimproved species-rich hay meadows located within a Site of Special Scientific Interest (SSSI). The 
experiment design comprised three replicate blocks, each of which contained four fields to which levels of 
nitrogen fertiliser between 25 and 200 kg per hectare had been added annually and a single reference field 
which had no application of nitrogen fertiliser. 
During the EISAC’89 experiment, image data were obtained with the Compact Airborne 
Spectrographic Imager (CASI) (see Jones, 1992), and a large body of ground data was collected. Reflectance 
data were collected using several spectroradiometers from the Natural Environment Research Council 
Equipment Pool for Field Spectroscopy (Rollin, 1991). This paper discusses bi-conical spectral reflectance 
data (0.35trm-23/tm) collected with a dual-beam GER IRIS MKIV spectroradiometer over Replicate Block 
3 on 21 May 1989. Time constraints limited the number of spectra collected at each sample location to a 
single spectrum at each of 6 randomly located quadrats in all 5 nitrogen treatments in Replicate Block 3, 
resulting in a data set of 30 spectra. At each sampling point, herbage was harvested from a 0.306m 2 quadrat 
and the following variables extracted: fresh weight, dry weight, percentage gravimetric water loss on drying 
and chlorophyll a and b. A measure of canopy water status was calculated as the gravimetric water content 
expressed as a percentage of the fresh biomass weight per square metre area of canopy. This is referred to 
here as wet% and is calculated as follows. 
wet weight per m 2 - dry weight per m 2 
wet weight per m 2 
* 100 
This analysis attempts to establish if useful relationships between spectral parameters and leaf water 
content can be identified in field data focusing primarily on the correlations between the spectral parameters 
and wet%. The contribution of canopy geometry and soil background is ignored on the grounds that grass 
canopy studied was complete and relative unif orm across all the sample plots, so the effect is likely to be 
insignificant. 
The analysis is confined to the 400-1360nm wavelength region which includes two water absorption 
features at 940nm and 1150nm. Other, major water absorption features occur in the SWIR; between 1400- 
1500nm and at 1920nm but outside the atmospheric windows ie. within the wavelength regions affected by 
atmospheric water vapour. This study focuses mainly on the 1150nm water feature. 
3. SPECTRAL DATA PROCESSING 
3.1. Pre-processing 
Pre-processing of the spectra required instrument specific calibrations to correct for dark current, gain, 
gratings overlap, target-reference detector function and wavelength. A further calibration was applied to 
correct for the reflectance of the BaS0 4 reference panel used as the reflectance standard. At this stage, data 
were output in a standard format comprising paired wavelength and absolute reflectance values as xy co 
ordinates. Efficient manipulation of high dimensionality data sets on a personal computer is beyond the 
capabilities of many general purpose statistical packages and was performed in this case using the signal 
processing package ASYST. This greatly simplified the process of correlating spectral data at all wavelengths 
with each of the biophysical parameters in turn and also facilitated advanced smoothing and data 
manipulations. 
3.2. Resampling 
Spectra measured with the GER IRIS MK IV have different sampling resolutions over the visible and near- 
infrared (NIR) wavelength ranges (400-1 lOOnm) and the shortwave-infrared (SWIR) region. In the region 
400-1100nm the resolution is just under 2nm compared with almost 4nm for wavelengths above HOOnm. 
Data within each wavelength region were dealt with separately and were initially resampled to a regular 
interval by fitting a cubic spline through the data points. For the visible/NIR wavelength range the spline 
was fitted through 440 points, resulting a modified spectral resolution of approximately 1.5nm. For the longer 
wavelength range a spline of 134 points was fitted used, resulting in a resolution of approximately 2.5nm.