ORYZA2000 v2.00 (May 2003) - Effect of drought computation

Two changes have been made in the computations of the effect of drought on crop growth and development. 

The effects of drought on partitioning is changed: 

!----------Relative growth rates of shoots and roots

!          Effect of drought stress on shoot-root partitioning

!BB: Changed according to SUCROS2

FSH = LINT2('FSHTB',FSHTB,ILFSHT,DVS)

FSH  = (FSH*CPEW)/NOTNUL((1.+(CPEW-1.)*FSH))

FRT = 1.-FSH

!           IF (DVS.LT.1.) THEN

!              FSH = FSH * LESTRS

!              FRT = 1.-FSH

!           END IF

The factor CPEW is used to represent the influence of water shortage on dry matter partitioning. When this ratio is above 0.5, the effect on physiological processes is usually small. Carbohydrate partitioning between shoot and root under water stress is altered in favour of the root biomass. Brouwer (1962) described the physiological principle of this mechanism, based on the functional equilibrium. Yet it is difficult to quantify the instantaneous growth response of root biomass to water stress. It is assumed that up to a moderate stress level, there is no significant effect on partitioning. At higher stress levels during the vegetative phase, the share that goes to the roots increases up to 50% of the amount that otherwise would have gone to the shoot. It is assumed that the relative partitioning of carbohydrates within the shoots between leaves, stems and storage organs is affected similarly to the partitioning between shoots and roots. The parameter CPEW (between 1 and 0) is used in the model as a multiplier in the calculation of the fraction of total dry matter increase allocated to the shoots (FSH).

In ORYZA2000 version 1, the drought-stress effect on leaf elongation rate, LESTRS, was also used to simulate the effect on partitioning. To separate these two, the variable CPEW is now also calculated in WSTRESS for lowland rice, and its value is the same as that of LESTRS. For aerobic rice, CPEW is calculated as the ratio of actual over potential transpiration (the same as PCEW).

The second change is the removal of the effect of drought on development rate: 

!----------Effect of drought stress on development rate

IF (DVS.LT.1.0) THEN

! BB: REMOVE THIS; IT CAN TAKE MORE THAN 1 YEAR TO COMPLETE A CROP CYCLE!!

!              DVEW = LESTRS + (DVS*(1.-LESTRS))

DVEW = 1.

ELSE IF (DVS.GE.1.) THEN

DVEW = 1.

END IF

DVR = DVR*DVEW 

There was no feedback mechanism on the reduction on development rate, and under moderate drought, it was found that development rates could come to a virtual stand-still without the crop dying.