Based on the results obtained from the field surveys, about 50% of the natural watercourse sub-sites were mapped accurately. The majority of the mis-identified sub-sites were actually modified watercourses. Such a change in the habitat type could have occurred after the satellite imagery and aerial photos used for the this Study were taken. Some of the watercourses were discovered to be wetlands (Sites 65b and 70c), mangrove (Site 65b) and grassland (Site 65a) (Table 6.1e).

As defined in TR1, those watercourses which were observed to have been heavily modified by pollution were mapped as Modified Watercourses. Adjustment of watercourse with regard to this aspect was based on the data from the EPD River Water Quality in Hong Kong (1998). EPD has graded Hong Kong river water quality into 5 grades, i.e., "Excellent", "Good", "Fair", "Bad" and "Very Bad" based on the monitoring results collected regularly from a series of river water quality monitoring stations. However, water quality is assigned to each monitoring station and not to the river reach immediately upstream or downstream of the station. In order to determine the water quality for each stretch of river between stations (and therefore assess those reaches which are of "Very bad" quality and therefore need to be mapped as modified watercourses), the following approach has been adopted to designating water quality to such reaches: from any monitoring station rated "Very Bad" in water quality downstream to the next monitoring station not rated "Very Bad" (or the sea) would be re-classified as Modified Watercourse.

Based on the river water quality data provided by EPD (1998), a total of 5 monitoring stations have registered "Very Bad" water quality (Table 6.1f). Except for Stations GR1 and GR2 (River Ganges in north New Territories), all the "Very Bad" river sections have already been identified as Modified Watercourse and thus no adjustment in habitat type for these streams is required. Based on the approach proposed above for mapping locations of river reaches, the whole downstream section of Station GR2 (until Station GR1) and downstream reaches of Station GR1 to the coast were re-identified as Modified Watercourse.

Another criteria for categorising a watercourse as "natural" or "modified" is based on the extent of water abstraction from the watercourse. Rivers with substantial abstraction of water for irrigation or domestic use were considered as modified watercourses (TR1). Based on the results obtained from field surveys, abstraction of water has been recorded in some of the natural watercourse sites but the extent of abstraction was not regarded as "substantial" enough to re-classify the habitat as Modified Watercourse (but this abstraction factor has been included as one of the conditions for ecological value adjustment) (see Table 7.1e).