Stockholm Environment Institute, Technical Report - 2012 Application of SWAT and a Groundwater Model for Impact Assessment of Agricultural Water Management Interventions in Jaldhaka Watershed: Data and Set Up of Models Devaraj de Condappa, Jennie Barron, Sat Kumar Tomer and Sekhar Muddu Application of SWAT and a Groundwater Model for Impact Assessment of Agricultural Water Management Interventions in Jaldhaka Watershed: Data and Set Up of Models Devaraj de Condappa, Jennie Barron, Sat Kumar Tomer and Sekhar Muddu Stockholm Environment Institute Kräftriket 2B SE 106 91 Stockholm Sweden Tel: +46 8 674 7070 Fax: +46 8 674 7020 Web: www.sei-international.org Head of Communications: Robert Watt Publications Manager: Erik Willis Layout: Richard Clay Cover Photo: This publication may be reproduced in whole or in part and in any form for educational or non-profit purposes, without special per- mission from the copyright holder(s) provided acknowledgement of the source is made. No use of this publication may be made for resale or other commercial purpose, without the written permission of the copyright holder(s). Copyright © March 2012 by Stockholm Environment Institute per cent ABSTRACT This study contributes to the understanding of potential for Agricultural Water Management (AWM) interventions in the watershed of Jaldhaka river, a tributary of the Brahmaputra river, located in Bhu- tan, India and Bangladesh. An application of the Soil Water Assessment Tool (SWAT) and of a simple lumped groundwater model was developed for the Jaldhaka watershed. The first stage of this work was to collect a large dataset to characterise the natural and agricul- tural contexts of the Jaldhaka watershed. The watershed has a contrasting topography, with mountains upstream and large plains downstream. It experiences high rainfall with a monsoonal pattern and an average of 3,300 mm/year. The river flow is seasonal, with a sustained flow during the dry season, high flows during the monsoon and recurrent flood events. The soils are sandy loam (upstream) to silty loam (downstream), with little permeability. The aquifers in the region are alluvial and the groundwater lev- els in the watershed are shallow and stable. This study contributed to the development of a precise landuse map which identifies the natural vegetation, the water bodies, the settlements / towns, the tea plantations and the different cropping sequences in the agricultural land. Agricultural statistics were gathered at administrative levels for cropping sequences and crop yields. The irrigation in the watershed is predominantly from groundwa- ter, with diesel pumps, to irrigate rice during summer and potatoes during winter. SWAT and the groundwater model were adjusted in an interactive manner: SWAT was calibrated against the observed streamflows while the groundwater model was calibrated against the observed groundwater levels and the interaction aimed at the convergence of both models. The performance was satisfactory for modelling the watershed on an average monthly basis. However, the model set-up failed to reproduce adequately the crop yields. This paper ends with a discussion of the modelling set- up and data collection for agro-hydrological modelling. This set-up was applied in an accompanying research report to study the current state of the hydrology in the Jaldhaka watershed and the impacts of two types of AWM scenarios. CONTENTS Abstract iii List of abbreviations viii 1 Introduction 1 2 Introduction to the modelling softwares 3 2.1 Soil and Water Assessment Tool (SWAT) 3 2.2 Groundwater model 3 3 Biophysical data of the Jaldhaka watershed 6 3.1 Digital Elevation Model 6 3.2 Streamflow data 6 3.3 Climate data 12 3.4 Soils 16 3.5 Groundwater data 20 3.6 Land-use 23 3.7 Agricultural 29 3.8 Irrigation 37 4 Modelling set up 40 4.1 Initial setting of SWAT 40 4.2 Calibration of the groundwater model and SWAT 45 5 Discussion 56 5.1 … on the input dataset 56 5.2 … on the model set up 57 6 Conclusion 58 Acknowledgements 60 Annex 62 References 70 LIST OF FIGURES Figure 1: Location of the Jaldhaka / Dharla river watershed (in purple). The delineation of the Jaldhaka / Dharla watershed were generated in this work. 1 Figure 2: Scheme of the modelling 3 Figure 3: Digital Elevation Model from the Shuttle Radar Topography Mission and locations where climatic and streamflow data was available 7 Figure 5: Topographic profile of the transect defined in Figures 3 and 4 7 Figure 4: Slope derived from the DEM, the two local meteorological and streamflow gauge stations 7 Figure 7: Available time-series for streamflows measured at Taluk-Simulbari and Kurigram stations, unfiltered (left) and average monthly streamflow, filtered (right); the vertical error bars indicate the statistical standard deviation of daily streamflows 9 Figure 8: Zoom around Kurigram on Google Earth where are visible the infrastructures for water diversion as well as the neighbouring rivers, in particular the massive Brahmaputra. 10 Figure 9: Representative average rainfall for the Jaldhaka watershed, as calculated by SWAT, and average streamflow at Kurigram (period 1998 – 2008) 11 Figure 10: Rainfall at Jalpaiguri and Cooch Behar stations (period 1988 - 2008). Top: daily rainfall. Middle: annual rainfall. Bottom: average monthly rainfall, the vertical error bars in red indicate the statistical standard deviation of daily rainfall (in mm/day) 13 Figure 11: Average climatic data at Jalpaiguri and Cooch Behar stations (period 1988 - 2008). Top: temperature. Middle: wind. Bottom: humidity. The vertical error bars indicate the statistical standard deviation of daily data 15 Figure 12: Distribution of the average annual rainfall in the sub-watersheds, as represented in SWAT (period 1998 - 2008) 16 Figure 13: The georeferenced soil map in the region of the Jaldhaka watershed 17 Figure 14: The Harmonised World Soil Database and its soil units in the region of the Jaldhaka watershed. 17 Figure 15: Plot in soil textural triangle of the United State Department of Agriculture 19 Figure 16: Location of the observation wells for groundwater level measurement. CGWB stands for Central Ground Water Board and SWID for State Water 21 Figure 17: Measured groundwater levels in the Jaldhaka watershed. In pale: level of different wells. In black: average of all the wells 22 Figure 18: Typical groundwater levels in the Jaldhaka watershed. The wells are located on Figure 16. The vertical error bars indicate the statistical standard deviation 22 Figure 19: Interpolation of average piezometric levels observed by the State Water Investigation Directorate (SWID) (period 1994 - 2009). 23 Figure 20: Satellite images acquired for high resolution landuse mapping. Note the demarcation between the north and south view 24 Figure 21: Location of the groundtruthing sites visited in April 2010 and draft unsupervised classification of the landuse. Right: zoom on the transect (note on this view the discrepancy 25 Figure 22: Calendar of the main cropping sequences in the Jaldhaka watershed 26 Figure 23: High resolution (10 m) landuse map of the Jaldhaka watershed (year 2008). 27 Figure 24: Photos of the spots identified on the landuse map (Figure 23) 28 Figure 25: Modified version of the landuse map (Figure 23, year 2008) entered in SWAT (90 m resolution) 29 vi Figure 26: Area of the major crops in administrative blocks containing the Jaldhaka watershed 32 Figure 27: Yield of the major crops in administrative blocks containing the Jaldhaka watershed. Mind the different vertical scale 33 Figure 29: Average monthly reference evapotranspiration calculated from difference sources 46 Figure 30: Calibration with respect to the actual evapotranspiration ETa. Monthly value of the different landuse vegetation categories (average over the calibration period, 1998 – 2008). 47 Figure 31: Piezometric levels simulated at a monthly time-step by the groundwater model vs. observations 48 Figure 32: Calibration with respect to the recharge of the shallow aquifer (GW_RCHG), average for the Jaldhaka watershed over the calibration period (1998 – 2008) 49 Figure 33: Calibration with respect to the shallow groundwater baseflow (GW_Q), average for the Jaldhaka watershed over the calibration period (1998 – 2008) 50 Figure 34: Streamflow simulated (FLOW_OUT) at Kurigram in the initial run over the calibration period (1998 – 2008) 51 Figure 35: Streamflow simulated (FLOW_OUT) in the final calibration (calibration run n°100) over the calibration period (1998 – 2008). 53 Figure A.1: Example of the groundtruthing form (site GT 35) filled by the field assistants 69 LIST OF TABLES Table 1: Topographic regions of the Jaldhaka watershed 7 Table 2: Available number of measurements at Taluk-Simulbari and Kurigram stations. Source of data: Bangladesh Water Development Board. 8 Table 3: Available climatic time-series and gaps in the datasets. RMC stands for Regional Meteorological Centre (Kolkata) and NCC for National Climate Centre. 12 Table 4: Annual rainfall at Jalpaiguri and Cooch Behar stations (period 1988 - 2008) 14 Table 5: Available measured groundwater levels in the Indian part of the watershed. CGWB stands for Central Ground Water Board and SWID for State Water Investigation Directorate. 20 Table 6: Distribution of the landuse categories (Figure 23) within the Jaldhaka watershed. 28 Table 7: Distribution of the landuse categories entered in SWAT (Figure 25). 30 Table 8: Available agricultural statistics. 30 Table 9: Average yields in the administrative blocks containing the Jaldhaka watershed,