In a greenhouse you may change the irrigation molality with a PEF 8000 in a treatment, whereas the salt one receives the same molality in NaCl; for doses and interactive terms you may follow https://www.jstor.org/stable/2557541?seq=1#page_scan_tab_contents for PEG values you may borrow an osmometer

Interesting ..Here is an abstract of one relevant study..

Abstract :The observations of a long-term experiment on the use of saline water were used to check thehypothesis whether models developed for drought conditions are valid for yield prediction in caseof salinity. The models for the relation between yield and evapotranspiration, proposed by Stewartand Hagan 1973 and Stewart et al. 1977 , were used for maize, sunflower, potatoes and Ž. Ž.soybean. The yield estimation for maize and sunflower was quite accurate, for potatoes somewhatless, but unsatisfactory for soybean. The estimated yield of soybean under saline conditions was higher than the measured one. This may be attributed to the differences in salt tolerance between soybean varieties or to an additional effect of salinity on the nitrogen supply of this legume. Source ; Katerji et al.Agricultural Water Management 36 1998 45–54...PDF enclosed for furtherr eading 

Dear Dr. Abdenour,

I suggest you to read the following paper for knowing appropriate methodology for conducting your research:

''Combined effect of water deficit and salt stress on ..... wheat ....'' by G. Semenova, I. Fomina and A. Ivanov (2014) Cell Biol. 3:14-24. 

For your easy access, I attach herewith the PDF copy of the paper.

Hope you find this input interesting and useful. 

Here is another very interesting work....Drought and salinity are among the major abiotic stresses which, often inter-relatedly, adversely affect plant growth and productivity. Plant stress responses depend on the type of stress, on its intensity, on the species, and also on the genotype.

Different accessions of a species may have evolved different mechanisms to cope with stress and to complete their life cycles. This study is focused on lentil, an important Mediterranean legume with high quality protein for the human diet. The effects of salinity and drought on germination and early growth of Castelluccio di Norcia (CAST), Pantelleria (PAN), Ustica (UST), and Eston (EST) accessions were evaluated to identify metabolic and phenotypic traits related to drought and/or salinity stress tolerance. The results showed a relationship between imposed stresses and performance of the cultivars. According to germination frequencies, the accession ranking was as follows: NaCl resistant > susceptible, PAN > UST > CAST > EST; polyethylene glycol (PEG) resistant > susceptible, CAST > UST > EST > PAN. Seedling tolerance rankings were: NaCl resistant > susceptible, CAST ≈ UST > PAN ≈ EST; PEG resistant > susceptible, CAST > EST ≈ UST > PAN. Changes in the metabolite profiles, mainly quantitative rather than qualitative, were observed in the same cultivar in respect to the treatments, and among the cultivars under the same treatment. Metabolic differences in the stress tolerance of the different genotypes were related to a reduction in the levels of tricarboxylic acid (TCA) cycle intermediates. The relevant differences, between the most NaCl-tolerant genotype (PAN) and the most sensitive one (EST) were related to the decrease in the threonic acid level. Stressspecific metabolite indicators were also identified: ornithine and asparagine as markers of drought stress and alanine and homoserine as markers of salinity stress.Source : Muscolo et al., : Journal of Experimental Botany

doi:10.1093/jxb/erv208

If I got your question correct, here you will find my answer. To get a specific level of water stress, you can use PEG 8000. You measure the osmolarity of the solution using an osmometer. Please note that the number 8000 is not a molecular weight since PEG is a polymer and it does not give straight line as you add the PEG to water or nutrient solution. as far as you get the desired osmolarity of PEG to induce specific water deficit, it is easy to get the equivalent NaCl osmolarity. Then, you can differentiate whether the effect is ionic or osmotic (i.e, water deficit). The following publication might of help for you to adjust the level of PEG and NaCl osmolarity: Mansour et al. (1993) J. Plant Physiol. 142: 623-628.