Generalized monsoon index (GMI)
GMI value was developed in 1982
as an agro-climatic index (Sukamoto et
al., 1984). It is a simple tool to monitor
rainfall conditions during the monsoon as
well as the overall crop conditions. The
Thai Meteorological Department uses
GMI value to assess the impact of rainfall
on agriculture in Thailand
(Meteorological Department, 2013). This
study used the GMI for comparison with
the SPI in the local watershed scale in the
Upper Ping River Basin (UPRB) to assess
the probability of water stress or over
wetness having an impact on crop
production in the future conditions under
the two climate scenarios (A2 and B2).
The value of GMI is calculated
from the monthly rainfall during the
southwest monsoon season. For Thailand,
the southwest monsoon season starts from
mid-May and culminates around mid-
October. Thus, the GMI of Thailand is
calculated by using the data on the
amount of rainfall from June to
September that is the rain fed growing
season under the influence of the
southwest monsoon, to be the value for
GMIsw. The weighting factor of the