Contents & Foreword, Characterizing And ... - IRRI books

mmmm450350250150ASoil moisture useSoil moisture accumulationSurplus waterRainfallPE500J F M A M J J A S O N D140120100806040200PercolationStanding waterFloodwaterJun Jul Aug Sep OctMonthMonthFig. 3. Climate water balance (A) and distributions of surplus water (B) at Jorhat, Assam, India.BTo develop management strategies, a detailed analysis of climatic water balancewas done for 18 years (1980-97). The depth of surface water in 80 well-distributedpaddy fields in four selected villages (20 fields in each village) representing thedistrict was measured twice a week from 2 June to 27 November (178 d duration) in1997, as sali rice crop cultivation and the monsoon in this region are confined to thisperiod.Sastri and Singh (this volume) give details of an agroclimatic inventory in Assam.Figure 3 shows the climatic water balance of bunded rice fields using average rainfalland PE values as inputs. This was computed using the bookkeeping procedure ofThornthwaite and Mather (1955). The following assumptions were made:1. The water-holding capacity of the soils up to saturation is 300 mm.2. From the surplus water that accumulates in the rice fields, percolation lossesoccur at the rate of 4 mm d –1 , or 120 mm mo –1 .3. After percolation, the excess water is stored in the fields as standing waterup to 50-mm depth in rice fields.4. Any water depth above 50 mm is considered as floodwater.The soil moisture recharge in Jorhat, Assam, starts on 18 March and continuestill mid-June (Fig. 3). From mid-June onward, percolation losses continue and standingwater up to 50-mm depth remains till 20 October. The flood period begins atJorhat after 15 June and ends by mid-August; its peak under normal conditions isaround mid-July. The average amount of floodwater in rice fields (above standingwater of 50 mm) is 112 mm.In all the villages, the water started accumulating from 4 June (24th SMW). Itsdepth fluctuated after 4 June and peaked on 13 August (33rd SMW) (Table 8). Then itdeclined and reached a minimum in October. The surface-water depth was directlyrelated to rainfall, which, during the peak water-depth period, was the maximum (307.6mm) of the season. Hydrology varied widely within the villages. Based on the sur-Planning and managing rice farming through environmental analysis 207