Water Productivity Of Watermelon With Deficit Mulched Drip Irrigation And Its Irrigation Scheme Decision-making In The Hexi Oasis Area

Located in the inland river basin of the northwest,the Hexi Oasis is a typical arid oasis agricultural area with resource-based water scarcity.It is an effective way to improve water productivity in this region by changing irrigation management techniques and optimizing water-saving irrigation schemes.Melon crops are relatively more adaptable to water than traditional crops,but the shortage of irrigation resources still restricts their production.Watermelon（Citrullus lanatus）,a crucial melon crop,has the advantage of a short growth cycle with high market demand and considerable economic benefits.To investigate the water productivity of watermelon with under-mulched drip irrigation and its water management strategies is of critical significance for the efficient cultivation of oasis melon crops.Regulated deficit irrigation,with the objective of saving water and increasing efficiency,is an essential irrigation technique for coordinating water inputs and production values in the oasis agriculture of the arid region,but there is relatively little application of water regulation deficit to oasis open-air watermelon,and further research is urgently needed.In this study,watermelon was used as the research object,and irrigation was applied by under-mulched drip irrigation.The 13 irrigation treatments were set up.Considering the actual local agricultural production,70%–80%FC（FC is the field capacity）was designed as full water supply,60%–70%FC as mild water deficit（WD）,and 50%–60%FC as moderate WD.Accordingly,single-stage WD treatments（N1 to N8）were:WD at the seedling stage（N1,mild;N2,moderate）,WD at the vine stage（N3,mild;N4,moderate）,WD at the expansion stage（N5,mild;N6,moderate）,and WD at the maturity stage（N7,mild;N8,moderate）,in that order;Multiple-stage WD treatments（N9 to N12）were:mild WD at both seedling and maturity（N9）,mild WD at seedling and moderate WD at maturity（N10）,moderate WD at seedling and mild WD at maturity（N11）,moderate WD at both seedling and maturity（N12）,and full water supply（70%–80%FC）control（CK）during the whole reproductive period,with a randomized group design.The field experiment was conducted from 2020 to 2021 at Yimin Irrigation Experiment Station,Minle County,Gansu Province,in the central part of the Hexi Corridor.We investigated the plant phenotype,photosynthetic physiological characteristics,water consumption characteristics,yield,and quality of watermelon,and analyzed the effects of various WD treatments on the growth and water utilization of watermelon.We also constructed a crop water production function and applied a multi-level integrated evaluation model to quantitatively determine the optimal deficit irrigation scheme.The main conclusions of the study are as follows:1)Deficit irrigation could significantly affect watermelon yield and quality under mulched drip irrigation conditions.Watermelon mild WD treatment at the seedling stage（N1）increased yield and water use efficiency（WUE）by 5.24–5.38%and 7.10%–7.37%,respectively,compared with CK,while WUE of mild WD treatment at both seedling and ripening stage（N9）in multiple-stages WD was similar to CK and not significantly different（P>0.05）.The WD treatment at the maturity stage significantly increased the soluble solids,soluble sugars,and Vc content of watermelon,and improved the flesh hardness to various degrees,with less effect on fruit diameter and volume.The overall quality of watermelon was evaluated employing principal component analysis,which indicated that treatments N9 and N11scored relatively high in terms of comprehensive quality.Additionally,the Blank and Jensen model was solved to show that the water sensitivity coefficients for watermelon are,in descending order of importance,the following:the expansion stage,the flowering and fruiting stage,the vine stage,and the seedling stage.2)Deficit irrigation significantly affected the growth dynamics indicators,dry matter accumulation,and translocation of watermelon.WD significantly（P<0.05）decreased the vine length,vine thickness,leaf number,and leaf area of watermelon at all stages of growth except for the maturity stage,with a relatively small decrease in the mild WD treatment,while the compensatory growth of WD treatment at the seedling or vine stage was obvious after rehydration,especially the mild WD treatment（N1）was always at the highest level after rehydration.WD at different stages caused a decline in plant dry matter accumulation,and the more severe the WD was,the greater the decline,with WD treatments at the vine and expansion stage significantly decreasing dry matter accumulation by 7.32%–14.10%and 7.60%–24.44%,respectively,compared to CK,while WD treatments at the seedling or maturity stage caused a relatively smaller decrease.Compared to CK,WD at pre-flowering significantly increased the root-to-crown ratio of plants during the deficit period and later,and the increase increased along with the degree of WD.3)Different levels of WD treatments at different growth phases significantly affected the photosynthetic physiological characteristics of watermelon leaves.Compared with CK,the moderate WD treatment at the seedling stage of watermelon significantly（P<0.05）reduced leaf net photosynthetic rate（Pn）,transpiration rate（Tr）,stomatal conductance（Gs）,and intercellular CO₂ concentration（Ci）,while the mild WD treatment had relatively smaller decreases in all indicators;The Pn,Tr and Gs of WD treatments at the vine stage were significantly decreased from 13.34%to 25.45%,14.02%to 27.22%and 8.58%to 25.07%,respectively,compared with CK,with the opposite trend of Ci in moderate WD treatments（N4）,while rehydration treatments（N1,N9,and N10）showed a compensatory but not significant difference from CK（P>0.05）;WD treatments（N5 and N6）at the expansion stage had similar effects on Pn,Tr,Gs,and Ci as in the previous stage.The leaf water use efficiency(L_WUE)and leaf intrinsic water use efficiency（WUEn）of the mild WD treatment at the seedling stage were not significantly different from those of CK,while L_WUE of the moderate WD treatment was significantly decreased;The L_WUE of the moderate treatment was slightly higher than that of the mild treatment after rewatering at the vine stage,while the L_WUE and WUEn of the WD treatments（N3 and N4）at this stage were not significantly different from those of CK and the rest of the treatments;The L_WUE and WUEn were basically at the same level for all treatments at expansion stage,while both L_WUE and WUEn were higher than CK for WD treatment at the maturity stage.Meanwhile,the leaf carboxylation rate（CE）of WD treatments at different periods except for the seedling stage decreased significantly due to the increase of WD.As can be seen,WD during the vine and expansion periods has a greater negative impact on photosynthetic physiological indicators of watermelon.4)WD under mulched drip irrigation affected water ecological characteristics of watermelon farmland.Water consumption of watermelon was significantly（P<0.05）affected by WD,with the highest total water consumption in CK（323 mm in 2020 and 331.41 mm in 2021）and a decrease in the rest of the treatments.Stage water consumption was highest at the expansion stage（about 40%of total water consumption）,followed by the vine stage,then the seedling and maturity stages,and lowest at the flowering and fruiting stage;The intensity of water consumption was the smallest at seedling stage(about 1.9 mm·d^-1),larger at vine and maturity stage(about 2.8 mm·d^-1),and the largest at flowering and fruiting stage(4–5mm·d^-1);Compared with CK,the WD treatment during various growing periods significantly reduced stage water consumption and intensity of water consumption,and the reduction increased with the degree of WD.The single crop coefficient method yielded the largest crop coefficient（mean value 1.16）at the watermelon expansion stage,followed by the flowering and fruiting stage（mean value 1.03）and the smallest at the seedling stage（mean value 0.45）.Besides,an exponential regression relationship between crop coefficients and leaf area index was established to estimate watermelon evapotranspiration.5)Deficit irrigation affected watermelon inter-root soil nutrients,microorganisms,and shallow temperatures.Post-harvest alkaline N,effective phosphorus,and fast-acting potassium contents of watermelon in all treatments decreased compared with pre-sowing,with the largest decrease in fast-acting potassium;The mild（N1,N9,and N10）WD treatments at the seedling stage had the lowest post-harvest levels of alkaline dissolved nitrogen,effective phosphorus,and fast-acting potassium,all of which were significantly（P<0.05）lower than those of CK.Mild WD treatments at the maturity stage of watermelon helped the growth of inter-root soil bacteria,while WD at all other periods inhibited bacterial production;WD at the seedling and vine stage significantly inhibited the production of inter-root soil fungi,while WD treatment at the expansion stage was favorable to fungi growth;Mild WD treatments at various stages were beneficial to the growth of actinomycetes,while moderate WD treatments at seedling and maturity stages had a relatively strong inhibitory effect on actinomycetes.At the beginning of the crop,WD treatments had little effect on post-harvest soil urease and sucrase activities,while WD at maturity inhibited urease activity and the more severe the WD,the greater the effect.At the beginning of the reproductive period,WD treatments had little effect on post-harvest soil urease and sucrase activities,while WD at the maturity stage inhibited urease activity,and the more severe the WD,the greater the effect.The daily variation of ground temperature in all reproductive periods with adequate water supply was lower than that in the WD treatment,but the variation of soil temperature decreased with the deepening of the soil layer and showed a lagging phenomenon.6)An integrated evaluation system was established for watermelon,and the best deficit irrigation scheme of watermelon with under-mulched drip irrigation was quantitatively determined in Hexi Oasis.A hierarchical model was established with 14 specific indicators belonging to four aspects of watermelon growth,yield,water consumption,and quality.The single-level weighted fuzzy evaluation resulted in the evaluation values,with N1 being the highest for both growth and yield indicators,N4 being the highest for stage water consumption indicators,and N8 being the highest for quality indicators;The results of the multilevel integrated evaluation based on gray correlation analysis（GRA）and distance between superior and inferior solutions（TOPSIS）showed that treatment N1 had a relatively high integrated score（GRA:0.079455,TOPSIS:0.090891）and treatment N9 was relatively second（GRA:0.079303,TOPSIS:0.094647）,which indicates that the comprehensive benefit of the watermelon is better with mild at seedling stage or mild at both seedling and maturity stages.In conclusion,deficit irrigation under mulched drip irrigation could regulate watermelon yield and quality,while for the Hexi oasis farming region,under the premise of water conservation,if only yield is considered,a mild deficit irrigation scheme at the seedling stage could be employed（N1）;if only quality is considered,a moderate deficit irrigation scheme at both seedling and maturity stages could be adopted（N12）;if both yield and quality are guaranteed,a multiple-stage deficit irrigation model（N9）with the mild deficit at both seedling and maturity stages can be preferred.The irrigation management strategy of mild WD（60%-70%）at both seedling and maturity stages while maintaining adequate irrigation（70%-80%）in other stages is recommended for open-air watermelon cultivation with under-mulched drip irrigation in cool irrigated areas of the oasis.