An index for plant water deficit based on root-weighted soil water content Jianchu Shi a,b,c , Sen Li a,b,c , Qiang Zuo a,b,c,⇑ , Alon Ben-Gal d a Department of Soil and Water Sciences, China Agricultural University, Beijing 100193, China b Key Laboratory of Plant–Soil Interactions, Ministry of Education, Beijing 100193, China c Key Laboratory of Arable Land Conservation (North China), Ministry of Agriculture, Beijing 100193, China d Soil, Water and Environmental Sciences, Agricultural Research Organization, Gilat Research Center, Mobile Post Negev 85280, Israel article info Article history: Received 6 August 2014 Received in revised form 16 December 2014 Accepted 17 December 2014 Available online 6 January 2015 This manuscript was handled by Corrado Corradini, Editor-in-Chief, with the assistance of Juan V. Giraldez, Associate Editor Keywords: Plant water deficit index Root distribution Soil water distribution Centralized signal Irrigation scheduling summary Governed by atmospheric demand, soil water conditions and plant characteristics, plant water status is dynamic, complex, and fundamental to efficient agricultural water management. To explore a centralized signal for the evaluation of plant water status based on soil water status, two greenhouse experiments investigating the effect of the relative distribution between soil water and roots on wheat and rice were conducted. Due to the significant offset between the distributions of soil water and roots, wheat receiving subsurface irrigation suffered more from drought than wheat under surface irrigation, even when the arithmetic averaged soil water content (SWC) in the root zone was higher. A significant relationship was found between the plant water deficit index (PWDI) and the root-weighted (rather than the arithme- tic) average SWC over root zone. The traditional soil-based approach for the estimation of PWDI was improved by replacing the arithmetic averaged SWC with the root-weighted SWC to take the effect of the relative distribution between soil water and roots into consideration. These results should be bene- ficial for scheduling irrigation, as well as for evaluating plant water consumption and root density profile. Ó 2014 Elsevier B.V. All rights reserved. 1. Introduction Increasing worldwide population and shortage of water make efficient use of water for agricultural crops imperative. Water movement within the soil–plant–atmosphere continuum (SPAC) is driven by potential gradients, first from soil into roots (absorp- tion), subsequently into leaves, and finally to atmosphere (transpi- ration). Plant water status is central to water flow in SPAC as the ‘‘bridge’’ between water supply and water demand, and thus an exact and timely evaluation of plant water status is critical for effi- cient irrigation scheduling (Jones, 2004). Plant water deficit occurs when root water uptake (RWU) cannot support atmospheric demand for transpiration. Plant water deficit index (PWDI), a dimensionless coefficient used to quantify the extent of water def- icit, has been defined as the ratio of water deficit to water demand (Thornthwaite, 1948; Woli et al., 2012): PWDI ¼ T p  T a T p ¼ 1  T a T p ð1Þ where T a and T p are actual and potential transpiration rates, respec- tively, cm d 1 . Calculating the ‘‘theoretical’’ PWDI with Eq. (1) is extremely challenging, especially under field conditions, since plant actual and potential transpiration rates are affected by many com- plicated factors and are difficult to determine. Two alternative approaches to estimate PWDI have been adopted in practice for irrigation scheduling. The first approach is based on plant water stress response, namely plant-based approach (PA). The PA, usually delineated by plant stress sensing parameters such as tissue water potential, plant growth rate or http://dx.doi.org/10.1016/j.jhydrol.2014.12.045 0022-1694/Ó 2014 Elsevier B.V. All rights reserved. Abbreviations: PWDI, plant water deficit index; SWC, soil water content; RLD, root length density; NRLD, normalized root length density; RWU, root water uptake; SPAC, soil–plant–atmosphere continuum; DAP, days after planting; SA, soil- based approach; PA, plant-based approach; TH, high water supply via traditional surface irrigation; TL, low water supply via traditional surface irrigation; SH, high water supply via subsurface irrigation; SL, low water supply via subsurface irrigation; SD, statistical distribution; LD, linear distribution; UD, uniform distri- bution; TPRPS, traditional paddy rice production system; GCRPS, ground cover rice production system. ⇑ Corresponding author at: Department of Soil and Water Sciences, China Agricultural University, Beijing 100193, China. Tel.: +86 10 62732504; fax: +86 10 62733596. E-mail addresses: shijianchu@cau.edu.cn (J. Shi), lisen0802@163.com (S. Li), qiangzuo@cau.edu.cn (Q. Zuo), bengal@volcani.agri.gov.il (A. Ben-Gal). Journal of Hydrology 522 (2015) 285–294 Contents lists available at ScienceDirect Journal of Hydrology journal homepage: www.elsevier.com/locate/jhydrol