Impacts of Root Distribution and Root Water Uptake on Maize Water Use in Shallow Groundwater Agroecosystems

Transcription

1 Impacts of Root Distribution and Root Water Uptake on Maize Water Use in Shallow Groundwater Agroecosystems Evren Soylu, Steve Loheide, Chris Kucharik LimnoTech Nelson Institute Center for Sustainability and the Global Environment Department of Civil and Environmental Engineering University of Wisconsin - Madison

2 Background Plant roots connect the soil to the atmosphere through helping transport water and nutrients residing in the soil to the tissues of plants. Root depth and the vertical profile determines the distribution and amount of water uptake

3 Plant drought adaptation strategies Shorter Time Scale (~Weekly) Root water uptake (RWU) compensation Hydraulic redistribution Seasonal Time Scale Developing fine roots based on the moisture availability (adapting their Root Length Density RLD) RLD = the total length of root per unit volume of soil

4 Root Water Uptake Compensation Transpiration Root Water Uptake

5 195! Agro-IBIS-VSF 196! response to a shortage Root Water Uptake Compensation Transpiration 197! decline in the photosyn 232! So that the sink term Si for the compensated Root Biomass RWU appr 198! assumed to uptake wate Root Water 233! calculated as: Transpiration Uptake Water Uptake 199! experiencing any water Fraction 255! 255! 234! Sii == TT FFWSWS2i2i!!!!!! 200! Compensated RWU i=1 i=2 i=3!!!!!! over the rooting profile Integrated Water Stress 256!!is!the!water!uptake!fraction!for!a!giv WS2iis 235! where!f where FWS2i the water uptake for( Rai mgiven soil FWS = å 256! where!f 201! fraction WS2i!is!the!water!uptake!fraction!for!a i). i Ri m i Root Adaptability Ri m i 202!! Factor = F max F, F. 257! 257! 236! FWS 2i = 237! 258! Using this approach, the Full compensated RWU Using!this!approach,!the!compensated!RWU!mec 204! RWU processes into a i mechanism Dry stress Anoxia Transpiration 238! 259! moisture throughout the root allowing in plants to d 205!zone, calculation Agro-IBI soil!moisture!throughout!the!root!zone,!allowing 258! FWS 2i ( ( value!is less! If the F!WS Stress Factor )).!!!! FWS 2 ( max ( FWS,203! FC )) transpiration. Unlike th WS 2 WS C Zone Zone Zone Using!this!approach,!the!compensated!RWU!m Feddes et al., ! Dry soil regions. Wet accounts for wa 206! already 239! soil!moisture!throughout!the!root!zone,!allow stressed 260! from!the!less<stressed!soil!regions.!

6 Model Description AgroIBIS-VSF T soil, θ, h LAI & stomatal C.

7 The influence of groundwater on land-atmosphere interactions In Case 2, water table is at critical depth, where small changes in groundwater depth cause significant vertical redistribution of soil moisture resulting in changes of water and energy balance at the land surface Kollet & Maxwell 2008

8 Objectives Transpiration Net Primary Productivity RLD root length RWU compensation How are seasonal scale (root length) and short-term (compensation) plant adaptive strategies able to mitigate the adverse effects of plant water stress during dry years? How does maize utilize groundwater given varying water table depths and how is the groundwater contribution influenced by the RLD distribution and RWU compensation mechanism?

9 Photo courtesy of Arlington Agricultural Research Station Simulation Design 1. Corn with 3 root distribution for a continuum of water table depths 2. Hourly meteorological observations between from the Arlington Agricultural Research Station 3. Compensated and uncompensated RWU approaches were applied separately 4. Separate simulations by applying and removing the effect of oxygen stress

10 Transpiration [mm] Transpiration [mm] Water Table Depth and Plant Water Use 350 With Oxygen Stress Comp Uncomp Root length Deep 300 Comp Uncomp 250 Comp Uncomp Without Oxygen Stress Water Water table table depth depth [m] [m] Comp Uncomp Shallow Deep Case I Case II Case III Water table depth [m] Comp Uncomp Comp Uncomp Intermediate Intermediate Shallow

11 Daily net primary productivity time series for an extreme dry year (2012) DROUGHT Compensation increases productivity

12 Deep Roots Typical Roots Shallow Roots Daily net primary productivity time series for an extreme dry year DTWT is kept constant at 2m As the roots become deeper effects of compensation are delayed and reduced

13 NPP contribution from compensated RWU NPP increase (g C m -2 year -1 ) 100 Shallow-Rooted Plants 80 Intermediate-Rooted Plants Deep-Rooted Plants Growing Season Precipitation (mm) NPP compensatory RWU NPP uncompensatory RWU

14 Spatial and Temporal Distributions of RWU igure 6. Plots show total plant RWU time series (black bars on top of each color maps) nd color maps show plant RWU distributions over the top 2 m of the soil column for lants with shallow RLD when a) constant head lower boundary condition was kept onstant at 2 m, b) free drainage condition is applied, and for plants with deep RLD when ) constant head lower boundary condition was kept constant at 2 m, d) free drainage ondition is applied for the year Groundwater at 2m has subtle effects on root water uptake in the top 50 cm RWU is better distributed across the domain when roots are deep and is less sensitive to drought

15 The groundwater subsidy decreases as roots become deeper because the deep rooted plants have access to more sufficient soil water even in the absence of shallow groundwater Influence of Root Depth and Compensation on Groundwater Subsidy to Transpiration Groundwater subsidy is additional water available for transpiration from shallow groundwater (Lowry and Loheide, 2010)

16 Summary & Conclusions Strength of the relationship between groundwater depth and plant water usage in the critical water table depth range is controlled by plant root structure and RWU strategies. The negative impact of drought on plants lessens as RLD becomes deeper Shallow rooted plants are more vulnerable to dry periods and their benefit is greater when GW is shallow The mean additional net carbon amounts that are fixed due to the compensated RWU approach were 5.1%, 5.5%, and 1.2% for the plants with shallow, typical and deep RLD, respectively.

17 Implications: Shallow groundwater exists globally Shallow groundwater is neglected in GCMs used to predict climate change although it alters the land surface energy balance From Fan et al., 2013

18 Photo courtesy of Sam Zipper Acknowledgements This material is based upon work supported by the National Science Foundation under the Water Sustainability & Climate Program (Grant No. DEB ). Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation. We thank the Arlington Research Station for data used in validation and as climate drivers.

19 NPP contribution for compensated RWU NPP compensatory RWU NPP uncompansatory RWU